  We explore a novel search strategy for dark matter in the form of massive
compact halo objects (MACHOs) such as primordial black holes or dense
mini-halos in the mass range from $10^{-4}$ to 0.1 solar masses. These objects
can gravitationally lens the signal of fast radio bursts (FRBs), producing a
characteristic interference pattern in the frequency spectrum, similar to the
previously studied femtolensing signal in gamma ray burst spectra. Unlike
traditional searches using microlensing, FRB lensing will probe the abundance
of MACHOs at cosmological distance scales (~Gpc) rather than just their
distribution in the neighborhood of the Milky Way. The method is thus
particularly relevant for dark mini-halos, which may be inaccessible to
microlensing due to their finite spatial extent or tidal disruption in
galaxies. We find that the main complication in FRB lensing will be
interstellar scintillation in the FRB's host galaxy and in the Milky Way.
Scintillation is difficult to quantify because it heavily depends on turbulence
in the interstellar medium, which is poorly understood. We show that,
nevertheless, for realistic scintillation parameters, FRB lensing can set
competitive limits on compact dark matter object, and we back our findings with
explicit simulations.
