[jpk]

Most of the study around dark matter is in the context of galaxies because that's where the gravity/light mismatch occurs. The rotation of galaxies (as influenced by the gravity of the matter in them) indicates there's more matter than what we can see via the light they emit.

That mismatch when observing galaxies is the whole reason we think dark matter is a thing.

[nyssos]

Most of the evidence is localized around galaxies (not just rotation curves, but also radial velocities and excess gravitational lensing), but not all. Even uniformly distributed dark matter would still show up as a contribution to the mass density of the universe, which curvature measurements indicate is much higher than baryonic matter alone can account for.