petercl14 said:
Have the scientists that believe in dark matter considered that the unexplained star orbits could in fact be due to the black holes?
Yes. But this possibility is heavily constrained, mostly to the point of being all but ruled out.
See, e.g.,
this Wikipedia article and the sources cited therein.
The kind of black holes most often suggested for this possibility are called "primordial black holes" and if they are the source of dark matter phenomena, they'd have to have masses similar to asteroids. There are no processes that can make black holes of this size now, but there might have been in the very early universe, so this is why they are called "primordial black holes." There is no observational evidence which has supported the existence of even a single particular primordial black hole.
In the time period after the very early universe, it takes a neutron star with a mass not less than somewhere between 2.14 and 3 solar masses (the exact cutoff is a matter of ongoing research) to be massive enough to collapse into a black hole and there is no other known method of black hole formation.
Small black holes are believed to evaporate over time in a process called Hawking radiation (and the smaller they are, the faster they evaporate), so only those large enough to not evaporate completely over 13 billion years or so would remain. Black holes larger than asteroids can be detected through "micro-lensing" of light in their vicinity.
Black holes which are stellar black holes or larger are predicted to absorb more matter from their surroundings including light, dust, and cosmic background radiation, than they would emit in Hawking radiation, so they don't evaporate.
petercl14 said:
Have they found even one black hole other than the one at the center of the milky way galaxy?
Yes. They have found many (more than 40)
"star sized" (a.k.a. stellar) black holes, and many
intermediate sized black holes which are typically 10-200 time the mass of the Sun (which is much smaller than the black holes at the center of galaxies which are called "
supermassive" black holes). Supermassive black holes are classically defined as black holes with a mass above 100,000 solar masses.
Intermediate sized black holes were only discovered definitively and in large numbers once gravitational wave detectors were invented and deployed. The first possible detection of one, in the year 2004, however, predated gravitational wave detection devices.
petercl14 said:
If they have then this would also confirm that there may in fact be 1000s of black holes as they also suggest. There would then be no need to bring in the idea of dark matter.
It would not. The number of stellar and intermediate sized black holes isn't large enough to explain the gravitational effects observed (there are too few of them by many orders of magnitude) and it is possible to rule out the possibility that we just aren't seeing enough black holes to cause the observed effects.
Mostly this ruling out is due to gravitational lensing, i.e. to the bending of light you would see if there was a black hole near the path of light that arrives on or near Earth. There isn't enough of that to involve enough black holes to account for dark matter.
There are also qualitative aspects of observed dark matter phenomena (like its tight alignment with ordinary mass distributions and wave-like apparent impact) which also disfavor black holes as a source of dark matter.
