  We present a model treating the energy-momentum relation of relativistic
particles as the upper branch of a generalized energy-momentum relation
emerging from a forbidden crossing between the constant energy of a massive
particle and the photon line. The lower branch, a regime dubbed as Einstein's
basement, gives rise to particles with different kinematics that is analysed in
the low-velocity limit. Allowing for gravitational interaction between those
particles, and between particles of both branches, we discuss whether particles
in Einstein's basement might be suitable dark matter candidates. Moreover, the
special dynamics of mixed systems leads to an expansion mechanism for regular
matter in space. Tests of the presented model by astronomical observations are
suggested.
