Nearly 100 years ago, we started  telling ourselves a particular story about the beginning of everything.

And it’s a story that starts with a  hot, unthinkably dense ball of matter, and ends with right now. You probably know it best as the Big Bang Theory.

And while different scientists may play  around with slightly different versions of it, it does a pretty solid job of explaining  all the stuff we see in the universe today. But there’s a lot of stuff we don’t see, too. Like, after the Big Bang Theory first came about, our picture of the universe expanded to  include a strange substance called dark matter, whose presence we can detect,  but not the stuff itself.

And so far, this mysterious form of matter  has just been folded into the Big Bang. Most theoretical astrophysicists  have carried on assuming that everything popped into  existence during that one moment. But what if this story we’ve told ourselves  for generations…about how stars and planets and galaxies came to be…isn’t true  for their dark matter cousins?

Recently, some scientists have proposed  that the Big Bang was followed by a “dark Big Bang,” which not only spawned  all the matter belonging to this dark sector of reality, but spawned more  matter than was made in the original. And if they’re right, studying  the Universe’s second, bigger bang could help us solve the  mystery of what dark matter even is. [♪ INTRO] Now before we can talk about the Dark Big Bang, we need to talk about the original Big Bang. In 1931, a Belgian scientist/priest  named Georges Lemaître made a simple conclusion.

Because humanity had recently discovered  that the universe was expanding, as opposed to just always being there,  and all he needed to do was hit rewind. The universe got smaller and smaller  until he hit the beginning of time. So Lemaître proposed a “primeval atom.” A super dense point that had the  whole universe packed inside it.

And some version of that has been  our universe origin story ever since. In the early days, scientists hypothesized  that the universe’s energy started off packed inside a singularity,  an infinitely dense, hot point. And in this version of the story,  the Big Bang was the moment when that singularity exploded  to create the universe.

That story explained the expanding  universe pretty well overall, but it came with some thorny problems. Like the fact that the very concept  of an infinitely dense point just… doesn’t exist physically. And if you take our best models of the universe and try to rewind the clock all the way to zero,  the whole thing breaks before it gets there.

The laws of physics spit out nonsense. So the modern concept of the  Big Bang is a little different. It puts the Big Bang after a brief period in  the universe’s ancient history called inflation.

In this version of the story, there was a  sliver of a second at the beginning of time when the universe expanded exponentially in size,  and it did so faster than the speed of light. No one knows what triggered this  inflation or what came before it. But within less than a billionth of a billionth  of a trillionth of a second, it was over.

And it was only then that all the  energy driving this breakneck expansion turned into matter and light,  giving way to the Big Bang, and giving rise to all the  stuff that is out there, today. The thing is, the picture gets a  little fuzzy when we try to extend it to the stuff we can’t see, which  makes up a big piece of the pie. For example, for every gram of regular,  visible matter, cosmologists believe that there are around five grams of so-called  dark matter…an invisible substance that, at least as far as we know, only  interacts with objects through gravity.

We can watch it make stars  at the edges of galaxies rotate faster than they would otherwise. And we can see how it warps spacetime,  bending light that passes by it. But we’ve never detected it directly.

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It’s part of what astronomers call the  dark sector, the part of the universe that’s made up of mysterious substances  that we’ve never explored directly, yet is most of reality as we know it. The dark sector makes up  around 95% of the universe, but all we know about it is how it pushes  and pulls on the rest of the universe… the part that’s made up of regular  matter and energy and light. So, the idea that everything in the dark sector was born during the Big Bang  is as good a guess as any.

But it’s still just a guess. There’s no actual reason  why it has to be that way. We can’t observe the moments  immediately after the Big Bang, because it was so hot and dense  that space was completely opaque.

Light couldn’t shine through the universe. So we don’t even know for sure  what regular matter was doing, let alone if dark matter was  there interacting with it. And, considering that regular and dark matter  barely interact with each other anyway, some scientists are considering whether dark  matter could have emerged at a later time, in what they call the Dark Big Bang.

It’s all hypothetical at this point. But as long as they’ve got this idea on the table, their task is to figure out how a Dark Big  Bang might have gone down if it did happen. It’s kind of like your lunch is missing, and  you’re trying to figure out if your dog ate it.

To solve the mystery, you might start by  imagining the potential clues to look for: For example, you know your dog is a messy  eater, so if it really did eat your BLT, it probably left some crumbs behind. Plus, it’s not supposed to eat human  food, so it might get an upset stomach. Or at the very least, it might not be  as hungry when dinnertime rolls around.

And astronomers are taking the  same general approach here. They’re trying to paint a picture  of the hypothetical Dark Big Bang so that observers can look for any  crumbs that suggest it really happened. And one crucial detail is when it happened.

Hypothetically, anyway. Cosmologists know that dark matter played a  big role in seeding the large-scale structure of the universe today: It helped draw  regular matter into the massive web of galaxy clusters that  stretches as far as we can see. So it had to have been around  early enough to play that role and fit into our models of  how that structure evolved.

And according to computer models, a dark big bang most likely took place  a mere month after the regular one. But a question that’s far more  important than the when is the what. What, if anything, actually happened?

The team proposing this dark  hypothesis suggests that, prior to this second “bang”,  there was a dark matter vacuum. Now a vacuum is what we  think of as pure emptiness. So in this dark matter vacuum there  would have been some regular matter, but it would have been empty of dark matter.

But according to quantum  mechanics, nothing is truly empty. Even so-called empty space is frothing  with particles that spontaneously… and incredibly briefly…pop  in and out of existence. So a vacuum still has at least  some amount of energy in it.

But that “at least” is important, because  a vacuum can have different energy states. While it likes to be in the  lowest energy state possible, better known as a true  vacuum, it doesn’t have to be. And scientists refer to that  situation as a false vacuum.

You can think of a false vacuum like  a pothole on an inclined street. If a ball gets stuck in the pothole,  whether it got kicked up the hill or was rolling down the hill, it  can stay stuck for a long time. But as soon as a car or a gust of wind  nudges the ball out of the pothole, it will roll down the street and wind up  resting at wherever the bottom of the hill is.

Because the bottom of the hill is at  a lower energy state than the pothole. Now, the energy state of a vacuum  is a little harder to picture, but the basic concept is the same: A false  vacuum may be stable for a long time, but it can suddenly drop to a lower energy state. And this is called a phase transition.

On Earth, we’re familiar with certain types  of phase transitions that happen in chemistry, like when liquid water vaporizes  into a gas or freezes into a solid. And cosmological phase transitions  aren’t completely dissimilar. They’re also sudden changes in  the form and properties of matter.

Some theorists hypothesize that  during the original Big Bang, a phase transition transformed a false  vacuum into a smattering of particles, not unlike how a gas condenses into a liquid. And in this hypothetical Dark Big Bang,  something similar may have happened, too. Just like a pot of water that’s beginning to boil, a false vacuum on the verge of  a phase change begins to bubble.

And, yes, the cosmologists  actually refer to them as bubbles. They’re little spots where the vacuum  starts dropping to a lower-energy state. And as they expand at near-light  speed and bump into each other, these bubbles collapse the  false vacuum into a true vacuum.

Now, in the case of the hypothetical  dark matter vacuum, the energy from these expanding bubbles gets released as dark  matter and dark radiation…much like the matter and light released in the collapse of  the vacuum that created the Big Bang. But exactly what comes out of this dark  phase transition depends on how it plays out. In one hypothetical scenario, bubbles  could slam together fast enough that their energy can turn directly  into humongous dark matter particles that are up to 10 trillion  times the mass of a proton.

The authors of this Dark Big Bang hypothesis have affectionately dubbed them dark-zillas. But if the phase transition plays out more slowly, the colliding bubbles will first  create a hot plasma in the dark sector. And while the plasma is hot,  small dark matter particles will emerge from interactions within it.

It’s this kind of transition that could  create the most popular candidates for dark matter particles out there, today. They’re known as WIMPS, or weakly interacting  massive particles, and scientists are actively searching for them with a bunch  of different experiments around the world. Eventually, the plasma cools, slowing  down the interactions taking place within the plasma, and ending  the creation of more dark matter.

But that’s just two scenarios of many. And right now, theorists aren’t  sure which, if any, might be right. But they hope they’ll find evidence  that points them in the right direction.

Because remember, even though this is all  hypothetical, and it all happened at a time we can’t observe directly, it’s like the dog that  ate your sandwich when you weren’t looking. The Dark Big Bang would have  left behind some crumbs. And while those bubbles that  carry out the phase transition between a false vacuum and  a real vacuum are invisible, they would have generated  gravitational waves as they collided.

Gravitational waves are ripples that  warp spacetime as they pass through it, subtly pinching and stretching  the distances between objects. And thanks to advancements in technology, scientists are getting better and better  at detecting tinier and tinier signals. Like in 2023, a massive international  collaboration called NANOGrav announced they had successfully detected a sort of gravitational wave background  noise coming from all around us.

Buried within that signal would be  gravitational waves created during the Big Bang. And eventually more detailed studies will help scientists tease  apart exactly what made them. And maybe one day, they’ll reveal  there really was a second Big Bang.

Or maybe they won’t. Cosmologists don’t need a Dark Big Bang to explain how we got a  universe full of dark matter. And there are plenty of alternate proposals  that I don’t have time to dive into, here.

Like primordial black holes, or a kind of  ultradense matter called “quark nuggets”. But I’m pulling for you Dark Big Bang, if only to read all the fun sequel  names that people put in the comments. [♪ OUTRO]