Early this year, a project called EDGES managed to take the Universe’s temperature at a time when the very first stars were forming. The results were somewhat confusing in that the Universe’s regular matter seemed to be much cooler than we’d expect, based on the energy it had shortly after the Big Bang. If the measurement is right, then something must have cooled the regular matter down.
Physicists immediately suspected dark matter, as every indication is that it’s relatively cold, meaning it’s moving slowly enough for gravitational interactions to control its behavior. But dark matter generally doesn’t interact with regular matter, making it hard to see how the two could have exchanged enough energy to cool the regular matter down.
Now, some physicists are back with a potential answer: a tiny fraction of dark matter has a charge, allowing it to interact with regular matter during the time between the Big Bang and formation of the Cosmic Microwave Background.