  Hypothesis of heavy stable quark of 4th family can provide a nontrivial
solution for cosmological dark matter if baryon asymmetry in 4th family has
negative sign and the excess of anti-U quarks with charge (-2/3) is generated
in early Universe. Excessive anti-U antiquarks form (\bar U \bar U \bar U)
antibaryons with electric charge -2, which are all captured by ^4He and trapped
in [^4He^{++}(\bar U \bar U \bar U)^{--}] O-helium (OHe) ``atom'', as soon as
He-4 is formed in Big Bang Nucleosynthesis. Interaction of O-helium with nuclei
opens new path to creation heavy nuclides in Big Bang nucleosynthesis. Due to
large mass of U quark, (OHe) ``atomic'' gas decouples from baryonic matter and
plays the role of dark matter in large scale structure formation with
structures in small scales being suppressed. Owing to nuclear interaction with
matter cosmic O-helium from galactic dark matter halo are slowed down in Earth
below the thresholds of underground dark matter detectors. However,
experimental test of this hypothesis is possible in search for (OHe) in
balloon-borne experiments and for $U$ hadrons in cosmic rays and accelerators.
(OHe) ``atoms'' might form anomalous isotopes and can cause cold nuclear
transformations in matter, offering possible way to exclude (or prove?) their
existence.
