[Salgak]

That assumes that it IS matter. While the most likely candidate for "dark matter" is, in fact, matter, there has been speculation on some exotic high-energy states which would act as matter from a gravitational standpoint. I can't claim to understand the underlying physics or its' mathematical expression, but a buddy of mine at NASA-Goddard who does astrophysics was telling me about it a month or three ago. . .

[jpsb]

The best guess on the nature of dark matter is black holes, "brown" stars, etc. No need for a new particle as this article implies.