If dark matter consists of a plethora of primordial black holes, then their gravitational lensing -- magnifying and brightening distant objects -- should be detectable. Physicists analyzed 740 known supernovas to find the handful that should have been magnified and brightened by black holes, and found none. This puts a strong upper limit, 40, on the percent of dark matter that can consist of black holes, and suggests that none of it is.