Stable matter of 4th generation: hidden in the Universe and close to   detection?
  Stable neutrino and U quark of 4th generation are excluded neither by experimental data, nor by astrophysical constraints. Moreover, excess of stable $\bar U$ quarks in the Universe can lead to an exciting composite nuclear-interacting form of dark matter, which can even dominate in large scale structure formation. 
K.Belotsky, M.Khlopov, K.Shibaev