Strictly speaking, black holes may not even be "matter", because, after the big scrunch that scientists think causes a black hole, all known laws of physics -- including the ones that define matter -- appear to be in abeyance. The news about black holes is that there appear to be many more than were expected, and some are also much nearer (but still at vast and safe astronomical distances) than expected. Although by definition black holes emit no light or radiation and therefore can't be seen, their gravitational effects, including brilliantly illuminated accretion disks and jets of anti-matter, are easily detected if you have the right equipment. And NASA has only recently orbited some of that equipment in the form of the Chandra X-ray telescope. Armed with new information from Chandra (and Hubble) and earth bound telescopes, scientists now also know where to look for more black holes: they seem to hang out in the center of spiral galaxies that have globular bulges in their middles -- like our own Milky Way galaxy. Although the view of any accretion disk from a black hole at the center of our galaxy is blocked by intervening stars, the characteristic jets of anti-matter appear to be spewing out of galactic center at right angles to the plane of the galaxy, just as would be expected. While black holes are incredibly dense and "weigh" a lot, the total of their masses seems to account for only about a tenth of the mass needed to explain the mechanics of the universe -- or "our" universe if you believe those scientist that seriously hypothesize other parallel ones like those that used to be only in science fiction.
For information on Chandra's new discoveries see http://fullcoverage.yahoo.com/fc/Science/Black_Holes/
To explain the movements of stars within galaxies, galaxies within clusters of galaxies, and clusters within super-clusters -- all recent discoveries -- there has to be much more mass, the other nine-tenths that black holes don't account for. On the smallest of these extraordinarily large scales are star movements in galaxies. Since scientists figured out, (only in the 1920's) that those spirally things that appeared to be at the edge of our galaxy were really whole separate galaxies, they assumed that, like terrestrial whirlpools, the galactic centers swirled faster and the spiral arms trailed behind. Recently, new observation platforms like the Hubble and Chandra space telescopes have enabled scientists to see individual stars in other galaxies, and super computers allowed the scientists to actually do the needed calculations. The scientists were surprised: whole galaxies appear to spin at the same speed and the spiral structures may exist because the spiral arms are stretching out ahead of the rotation of the centers. The only way that astrophysicists can thus far account for this is that there must be a lot more matter outside the visible galaxies. And, to produce the observed star motion, it has to be moving in the galactic plane. And the invisible stuff has to extend vast distances beyond what can be seen. Galactic disks would have vast amounts of this "dark matter" and the edges of galaxies would be crunching up against each other. Our own galaxy, if the calculations are correct, is already in collision with our closest neighbor, Andromeda. There may be even more of this dark matter that controls the formation of the observed galaxy clusters and of clusters of galaxy clusters called super-clusters, but computers haven't gotten big enough yet to do the calculations. So why can't we see all this stuff? First, any dark matter at the edge of our own galaxy is in the same plane as all the stars between us and the edge and is blocked from view. Other galaxies are too far away for us to see, with any present equipment, anything that is not glowing intensely, and dark matter is too thinly spread to support the nuclear reactions that would make it "bright matter", i.e., stars.
For more on dark matter, http://www.sciencenews.org/20000108/bob3.asp,