Scientists can only detect 5 percent of the matter in the universe. That 5 percent is everything you know: planets, stars, galaxies, selfie sticks. The rest is "dark" — stuff that can't be directly observed, but gives hints of its existence. 68 percent of the universe is dark energy, which is an invisible force that's probably driving the universe's expansion; the final 27 percent is dark matter, which exists in and around galaxies and exerts a strong gravitational pull. Scientists still haven't been able to prove that either of these "dark" elements exist, but they have some ideas for what they could be and how to detect them.

We know what the universe was like right after the Big Bang because we can see echoes of it in leftover thermal radiation, or what's known as the cosmic microwave background (CMB). The CMB isn't uniform; some spots are hotter or colder than others, and scientists use those temperature fluctuations to calculate the density of the dark matter in our very young universe. That, in turn, tells us other things about what things were like back then, such as how fast the universe was expanding and even how fast light traveled at the time.

Here's the thing: Just as if you left your car in the driveway and expected to see the same number on the odometer when you got back, scientists expect to be able to take those measurements of the early universe and figure out how things should look right now. Strangely, things look a lot different than they should. The universe's rate of expansion is off by about 10 percent.

"It's not a strong enough discrepancy to call it a crisis, but something may be going on," Torsten Bringmann at University of Oslo in Norway told New Scientist. "And it is not going away. It's stuck with us for the last 2 or 3 generations of experiments."

If this isn't some persistent error in the numbers and the early universe truly was that far off from what we see today, then it's up to scientists to figure out why. One leading explanation is called the decaying dark matter (DDM) hypothesis, which says that if today's universe doesn't have as much dark matter as it did then, that might explain the differences between then and now.

In 2016, Russian scientists tested this hypothesis by creating a model that would show what the universe might look like if the dark matter was disappearing. They calculated that the universe today has about 5 percent less dark matter than it did in its infancy. In 2018, Bringmann and his team created similar models with similar results, concluding that "a few percent" of the universe's dark matter has decayed into undetectable "dark radiation."

This is a crucial detail in the search for dark matter. So far, there are a few theoretical particles scientists have come up with as leading candidates for dark matter and, of course, each of those particles has particular characteristics. If dark matter has the ability to decay — or if it's decaying as we speak — that changes the characteristics scientists need to be searching for. "It'd immediately imply that a whole class of ... dark matter scenarios would be ruled out," says Bringmann. That narrows the scope of what we're looking for, bringing us one step closer to finally figuring out what dark matter really is.