Primordial Black Holes may be Dark Matter of the Universe Room: 6620 Primordial black holes (PBHs) are black hokes which are believed to have formed in the very early Universe. Recently an idea that PBHs may be dark matter of the Universe is attracting a lot of attention. In this talk, I first briefly review our knowledge about dark matter and black holes, then introduce the idea of PBHs as dark matter of the Universe, and discuss how it can be observationally tested. It turns out that gravitational waves are the key to this observational test.