  We propose a novel dark matter (DM) scenario based on a first-order phase
transition in the early universe. If dark fermions acquire a huge mass gap
between true and false vacua, they can barely penetrate into the new phase.
Instead, they get trapped in the old phase and accumulate to form macroscopic
objects, dubbed Fermi-balls. We show that Fermi-balls can explain the DM
abundance in a wide range of models and parameter space, depending most
crucially on the dark-fermion asymmetry and the phase transition energy scale
(possible up to the Planck scale). They are stable by the balance between
fermion's quantum pressure against free energy release, hence turn out to be
macroscopic in mass and size. However, this scenario generally produces no
detectable signals (which may explain the null results of DM searches), except
for detectable gravitational waves (GWs) for electroweak scale phase
transitions; although the detection of such stochastic GWs does not necessarily
imply a Fermi-ball DM scenario.
