How the universe’s expansion may have stopped dark matter taking over

Dark matter is a core component of the universe, but how did we end up with exactly the amount needed to hold the universe together? Physicists have proposed a new mechanism where dark matter particles in the early universe converted regular matter into dark matter exponentially, before being slowed down by the expansion of the universe.

Physicists estimate that dark matter particles outnumber regular matter by a ratio of five to one, and that dark matter plays a key role in the large-scale structure of the cosmos. Not only did its gravitational influence seed stars and galaxies to form in the first place, but it still holds galaxies and clusters together today. Without this specific density of dark matter, the universe would have evolved along very different lines.

So how did we get to this dark matter density? For the new study, a team of researchers proposed a new mechanism that they say is relatively simple and can be tested with future observations.

Many models suggest that dark matter was born from a “thermal bath,” the primordial plasma of regular matter particles in the early universe. From there, the team’s new hypothesis follows what’s known as a freeze-in model – essentially, the idea is that there wasn’t much dark matter to begin with, but the thermal bath of regular particles created dark matter particles over time, until it reached the density we see today.

But the team’s model adds a new part to the story: dark matter particles can convert regular particles into more dark matter. This new dark matter can then turn more regular matter to the dark side, leading to exponential growth of dark matter.

Of course, that model would eventually lead to a universe dominated by dark matter, but there’s an intriguing self-limiting mechanism already built into our understanding of the cosmos: expansion. Dark matter levels can grow very quickly in the early universe because regular matter was extremely dense at the time, but as the universe expanded and matter spread out, there was less fuel for the process and it slowed down.

The researchers showed that the model works to explain the current dark matter density, and can work with a range of dark matter particle masses. It also helps patch up holes in other models, that otherwise work well to explain observations.

Importantly, the idea can be tested. The team says that this mechanism would leave a specific fingerprint on the cosmic microwave background radiation, which could be spotted (or ruled out) by future observations.