  Fermion dark matter particles can aggregate to form extended dark matter
structures via a first-order phase transition in which the particles get
trapped in the false vacuum. We study Fermi balls created in a phase transition
induced by a generic quartic thermal effective potential. We show that for
Fermi balls of mass, $3\times 10^{-12}M_\odot \lesssim M_{\rm FB} \lesssim
10^{-5}M_\odot$, correlated observations of gravitational waves produced during
the phase transition (at SKA/THEIA/$\mu$Ares), and gravitational microlensing
caused by Fermi balls (at Subaru-HSC), can be made.
