  We study the properties and phenomenology of particle-like states originating
from D-branes whose spatial dimensions are all compactified. They are
non-perturbative states in string theory and we refer to them as D-matter. In
contrast to other non-perturbative objects such as 't Hooft-Polyakov monopoles,
D-matter states could have perturbative couplings among themselves and with
ordinary matter. The lightest D-particle (LDP) could be stable because it is
the lightest state carrying certain (integer or discrete) quantum numbers.
Depending on the string scale, they could be cold dark matter candidates with
properties similar to that of wimps or wimpzillas. The spectrum of excited
states of D-matter exhibits an interesting pattern which could be distinguished
from that of Kaluza-Klein modes, winding states, and string resonances. We
speculate about possible signatures of D-matter from ultra-high energy cosmic
rays and colliders.
