Why can't dark matter be black holes?

Question

Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we observe?

Answer 1

Even quiescent black holes tend to show up, through microlensing. Observational tests have put pretty rigorous constraints on a range of black holes masses in the Milky Way, although intergalactic black holes are not as well constrained.

The other problem is figuring out how you make lots of black holes, especially at smaller scales. That's not to say that scientists aren't still coming up with ideas, though. Warning: shameless plug. Further warning: We never found any empirical confirmation, so until that happens, it was just a promising idea that didn't pan out.

Answer 2

Part of the reason is a matter of ratios.

We're not talking about a fraction of the total mass that can be observed. We're talking multiples. There's about 10x as much dark matter in any large galaxy than there is normal matter that can be observed. If this was all in black holes, then there would be 10x as many black holes as there are stars. That would be pretty obvious actually, as those black holes would interact with other matter in the galaxy (in say, nebulae, both cold and hot), and the effects would be quite obvious and catastrophic. For one, it would make life on earth impossible, as there should be around 10 (give or take a few) black holes hovering around our sun at less than interstellar distances (just by the law of averages), probably spewing x-rays and subatomic particles our way. Nevermind the major gravitational effects they would have on the formation of planets. We're pretty sure that's not happening.

Also, the velocity curves of galaxies where velocities of stars is graphed vs distance from the galactic center, demonstrate that the mass of the galaxy is quite evenly distributed. This would not be the case if the proposed black holes were either exceptionally massive (as with the black hole at the center of most or perhaps all galaxies) or even not-quite-so-massive-but-evenly-distributed.

Instead, dark matter seems to completely lack interaction with other matter except by gravity. This effect can be seen in colliding galaxies, where the galaxies' dark matter halos appear to keep moving while the normal, detectable matter gets left behind.

Answer 3

Black holes are relatively easy to see as they emit high energy radiation.

And even when there is nothing falling into a black hole to produce radiation, light is bent round a black hole, so makes it visible.

Dark matter isn't easy to see, in fact it is incredibly hard to identify - which is why the debate continues as to how much there is.