Well, any other theory would have to explain all of the following better than cold dark matter does:
1. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.
1. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.