What if Dark Matter is just that? Dark matter. If there were lots and lots of something like planets out there, which for one reason or another don't reflect light, that would be something dark right?
Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility?
In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?
Black holes are indeed[1] a dark matter contender!
Just because the best explanation[2] we have right now is cold dark matter (CDM), doesn't mean physicists are content. They're looking for new ways[3] to probe not just CDM but other dark matter models as well, like primordial black holes[4].
[2]: http://pirsa.org/20040087/ "Dark Matter: A Cosmological Perspective", nice colloquium talk giving overview of what we know about dark matter.
[3]: http://pirsa.org/20100052/ "Strong gravitational lensing as a cosmological probe: new ways of constraining dark", random technical talk about new way of probing CDM.
Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)
They have actually looked for black holes of many sizes and have ruled out most of the range. It might still be possible, though improbable because they looked very hard for black holes.
Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility?
In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?