  Are some neutron stars produced without a supernova, without ejecting mass in
a remnant? Theoretical calculations of core collapse in massive stars often
predict this. The observation of the repeating FRB 121102, whose dispersion
measure has not changed over several years, suggests that dark core collapses
are not just failures of computer codes, but may be real. The existence of one
repeating FRB with unchanging dispersion measure is not conclusive, but within
a decade hundreds or thousands of FRB are expected to be discovered, likely
including scores of repeaters, permitting useful statistical inferences. A
na\"{\i}ve supernova remnant model predicts observable decline in dispersion
measure for 100 years after its formation. If an upper limit on the decline of
2 pc/cm$^3$-y is set for five repeating FRB, then the na\"{\i}ve model with
nominal parameters is rejected at the 95\% level of confidence. This may
indicate dark neutron star formation without a supernova or supernova remnant.
This hypothesis may also be tested with LSST data that would show, if present,
a supernova at an interferometric FRB position if it occurred within the LSST
epoch.
