  The nonbaryonic dark matter of the Universe is assumed to consist of new
stable particles. Stable particle candidates for cosmological dark matter are
usually considered as neutral and weakly interacting. However stable charged
leptons and quarks can also exist hidden in elusive dark atoms and can play a
role of dark matter. Such possibility is strongly restricted by the constraints
on anomalous isotopes of light elements that form positively charged heavy
species with ordinary electrons. This problem might be avoided, if stable
particles with charge -2 exist and there are no stable particles with charges
+1 and -1. These conditions cannot be realized in supersymmetric models, but
can be satisfied in several alternative scenarios, which are discussed in this
paper. The excessive -2 charged particles are bound with primordial helium in
O-helium atoms, maintaining specific nuclear-interacting form of the dark
matter. O-helium dark matter can provide solution for the puzzles of dark
matter searches. The successful development of composite dark matter scenarios
appeals to experimental search for doubly charged constituents of dark atoms.
Estimates of production cross section of such particles at LHC are presented
and discussed. Signatures of double charged particles in the ATLAS experiment
are outlined.
