Re: Could there be particles that only interact gravitationally?

Yevgeniy,

There are no observed, confirmed particles that interact only gravitationally. This might be because there really are no particles that interact only gravitationally, or because the gravitational interaction is so weak that observations are incredibly difficult.

There is nothing in principle that would forbid particles from interacting only gravitationally. If you subscribe to T.H. White's dictum in The Once and Future King, "Anything not forbidden is compulsory," then such particles must exist.

Candidates for particles that interact only gravitationally include: the graviton, the hypothetical quantum of the gravitational field; mirror matter (see link below); and, if E8 x E8 heterotic string theory correctly describes the Universe, shadow matter. The last two are dark matter candidates (see link below). The graviton, if it exists, would be massless. There are no constraints on the masses of the other forms of matter.

References:
http://en.wikipedia.org/wiki/Graviton
http://en.wikipedia.org/wiki/Mirror_matter

E8 x E8 matter
http://www.sukidog.com/jpierre/strings/susy.htm
http://en.wikipedia.org/wiki/Dark_matter

--Dr. Randall J. Scalise http://www.physics.smu.edu/scalise