The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact. Despite the indirect evidence for dark matter, we’d love to be able to detect it directly, which is something that can only happen if there’s a non-zero cross-section between normal matter and dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

Cold Dark Matter Is Heated Up By Stars, Even Though It Cannot ‘Feel’ Them

If dark matter doesn’t interact with normal matter or light, how can it be heated up?

One of the great cosmic mysteries of our time is the presence and existence of dark matter. Unlike normal matter, which is made of known particles that can emit, absorb, or otherwise interact with light and the other known particles, dark matter simply passes through both itself and everything else. It is completely invisible, as far as we can tell, except for one effect: it appears to have a gravitational mass. It affects the curvature of spacetime, and holds galaxies, clusters of galaxies, and the great cosmic web together.

When we run our simulations, however, we get very specific predictions for the structures that dark matter should form. The cosmic web lines up, but the smaller, galactic scales don’t. Long touted as the biggest problem for cold dark matter, scientists have uncovered the solution: dark matter gets heated up by stars. Here’s the story of how that happens.

At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Although normal matter and antimatter particles can collide with themselves and with radiation, dark matter particles should simply pass through one another without interacting. (BROOKHAVEN NATIONAL LABORATORY)

Imagine the Universe as it might have been in the earliest stages after the Big Bang…