From what I've read, given the speed of stars rotating within a galaxy and the known mass, the stars' speed is too fast to stay in orbit. Dark matter was proposed to explain this but is it possible that there are undiscovered smaller black holes with no orbiting stars?
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$\begingroup$ Yes, this has been thought about a lot, the term is "machos". Well explained in the answer below. $\endgroup$– FattieDec 9, 2016 at 2:32
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MACHOs (massive compact halo objects) are thought to make up a portion of the dark mass and portion of these are thought to be black holes, but there are several very solid reasons why black holes not generally seen as a serious alternative explanation to non-baryonic dark matter:
1) Observed elemental abundances are not consistent with the amount of baryonic matter being produced in the early Universe to explain the dark mass
2) Microlensing searches for MACHOs of the size of black holes formed by stellar evolution rules them out as making a significant amount of dark matter in the Milky Way.
3) Black holes are observable through accretion. For black holes to make a significant amount of dark matter they would have to be in isolated locations within the galaxy so as not to be visible. However most dark matter must be distributed evenly throughout the Milk Way to explain the rotation curve.
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$\begingroup$ Thanks for the detailed response! I have a couple follow up questions 2) How much of the milky way was observed for microlensing? I did a basic search for it en.wikipedia.org/wiki/Gravitational_microlensing and it seems like it's a very rare event because everything needs to line up properly 3) Would it be possible for there to be lots of black holes with nothing around it? Either the planets orbiting were blown away during the supernova event or most of the planets were orbiting very close during the super nova event $\endgroup$ Dec 8, 2016 at 3:44
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$\begingroup$ @AndrewPark You'd still see microlensings, and observations tell us that dark matter is everywhere. Especially all around us and around and within this galaxy. So it'd have to be an extraordinarly bizarre situation for the dark matter "here" to be meaningfully different from intergalactic dark matter. Also, to rule out microscopic black holes, some astronomers showed that they would pass through stellar cores, and would induce supernovae in otherwise stable compact objects. The more of dark matter they make up, the more obvious and detectable it becomes. But it's neither, by observation. $\endgroup$ Dec 8, 2016 at 4:28
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$\begingroup$ I think point number 2 is the strongest evidence against MACHOs. We can statistically predict how often we should see microlensing events based on what the dark matter content must be if it is made of MACHOs and we just don't see what the math tells us we should see. So that leads us to believe MACHOs are not (a major part of) the answer. $\endgroup$– zephyrDec 9, 2016 at 3:30