A certain type of supernova (I believe called type Ia, "one a") always have the same brightness when one explodes. Hence we can tell how far away one is by measuring its brightness (brightness falls off with the square of distance--one must account for other factors, such as dust absorbing the light). By measuring the distances to very distant supernovae and measuring the speed with which they are moving away from us, astronomers have concluded that the rate of expansion of the universe is not slowing as it would if only gravity were acting. There must be something counteracting gravity and pushing distant galaxies away from each other--the cosmological constant, or quintessence, or dark energy.