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. There’s no evidence for that, nor for a changing relative abundance between dark and normal matter. (Credit: Illustris Collaboraiton/Illustris Simulation)

Why modifying gravity doesn’t add up

The Universe gravitates so that normal matter and General Relativity alone can’t explain it. Here’s why dark matter beats modified gravity.

When we look out at the Universe — from the Moon, planets, and objects in our Solar System to the stars, galaxies, and even grander structures — we assume that all of these systems obey the same fundamental laws. We also assume that the full suite of what we observe can be explained by the same sets of particles that govern our own existence. Unfortunately, at least one of these two assumptions must be wrong, as applying the known laws of physics to the particles of the Standard Model known to exist cannot explain the full suite of structures and behaviors that we observe.

It’s long been realized that adding just one additional ingredient to the Universe can explain the behavior of all the structures that we see. That ingredient, known as dark matter, would have the following properties:

• it would always be cold, or moving slowly compared to the speed of light,
• it would exist in five times the abundance of normal matter,
• it would gravitate, but wouldn’t experience the electromagnetic or nuclear interactions,
• it wouldn’t collide…