Black holes are obvious answer to dark matter, why is that wrong? Primordial Black Holes as dark matter candidate. 
I think this is interesting and new, in that there is a window of PBH masses (link from Count Iblis) And the first GW event from Ligo is right in that window.  
This seems to be back in the news, (Well my latest copy of science news) 
Searching brings up previous papers (all behind pay walls.)
There have been some previous question here, and, more.
This seems like a nice solution, I'm mostly looking for other arXiv articles on the subject.  Or reasons why it can't be so.  
I edited my title to be a question.
Here's a link from one of my references.  
 A: As the article points out, there is a window between 20 to 100 solar masses available for black hole dark matter to evade the microlensing constraint mentioned in Sean Lakes's answer. Now, the possibility of primordial black hole dark matter has traditionally been considered for very small black hole masses. The idea was that after the big bang small mass black holes could have formed. Black hole masses that are very light, the mass of the Earth or lighter, would be strong gamma ray emitters via the Hawking process. If most of the dark matter mass were in the form of such light black holes, then that would inconsistent with the observed gamma ray background, so we can rule that out.
Larger mass black holes are ruled out via the microlensing constraint, but this leaves open the window between 20 and 100 solar masses. The question is then how such black holes could have formed. You cannot start out with the conventional low mass primordial black holes to get most of the dark matter mass in the 20 to 100 solar mass window via merger processes. But there are possible mechanisms to directly form heavy black holes during the inflationary era, see e.g. this article. 
A: The class of dark matter theory you're talking about are called Massive Compact Halo Objects (MACHOs). The short version is: when a MACHO passes in front of a star in a background galaxy, primarily the LMC and SMC, the gravity of the MACHO will lens (or microlense) the light, causing a brief magnification. When we watch for events like this, we don't see them as often as we'd expect for dark matter to be made from objects in the range of masses that astrophysical processes we understand can produce.
