The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact, as well as the initial density fluctuations that have their origin in quantum physics. The structures that arise, including galaxy clusters and larger-scale filaments, are indisputable consequences of dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

Ask Ethan: Can Dark Matter Really Explain The Universe’s Structure?

Why does dark matter, if it doesn’t dissipate energy, become gravitationally bound at all?

One of the most puzzling components of the Universe has to be dark matter. Although we have extraordinary astrophysical evidence that the normal matter in the Universe — the stuff made out of known particles in the Standard Model — cannot account for the majority of the gravitational effects we observe, all of that evidence is indirect. We still have yet to obtain a shred of repeatable, verifiable direct evidence for whatever particle might be responsible for dark matter. The total evidence places very tight constraints on any non-gravitational interactions that dark matter might possess. But if dark matter only interacts via the gravitational force, can it really explain the Universe’s structure? That’s what Patreon supporter Dr. Laird Whitehill wants to know, asking:

“If dark matter particles don’t interact and the only force that governs their motion is gravity, how do dark matter particles coalesce into a cloud? [In other words,] why aren’t all the particles hyperbolic?”

This is a very deep question, and the answer takes us deep into the heart of how gravity works in the Universe. Let’s start in our own backyard.