The possibility that part of the dark matter is made of massive compact halo objects ( MACHOs ) remains poorly constrained over a wide range of masses , and especially in the 20 - 100 M _ { \odot } window .
We show that strong gravitational lensing of extragalactic fast radio bursts ( FRBs ) by MACHOs of masses larger than \sim 20 M _ { \odot } would result in repeated FRBs with an observable time delay .
Strong lensing of a FRB by a lens of mass M _ { L } induces two images , separated by a typical time delay \sim few \times ( M _ { L } / 30 M _ { \odot } ) milliseconds .
Considering the expected FRB detection rate by upcoming experiments , such as CHIME , of 10 ^ { 4 } FRBs per year , we should observe from tens to hundreds of repeated bursts yearly , if MACHOs in this window make up all the dark matter .
A null search for echoes with just 10 ^ { 4 } FRBs would constrain the fraction f _ { DM } of dark matter in MACHOs to f _ { DM } \lesssim 0.08 for M _ { L } \gtrsim 20 M _ { \odot } .