A one-time explosion is not very satisfying, theoretically. So, a cyclic series of infinitely many big bangs is preferred. But, if it keeps expanding, then it only happened once. The universe that we see is not massive enough to ever begin contracting again. One reason is that without this missing mass, clusters of galaxies would have flown apart, by now.
Cosmologists continue to search for the missing mass, with both theory and observation.
See Hubble's Law . So the universe is either finite, or expanding, or both. This is a graph showing the brightness of stars along one axis, and their colors along the other axis. The use of this diagram has payed off in many ways, including helping to estimate the distance to stars. In many cases, it gives strong clues to the evolution of stars, and can help estimate their ages.