There's actually nothing in science that says that the universe
was a singularity. What is known is this: The universe is homogeneous and isotropic on large scales, so if we want to use general relativity to describe the large-scale behavior of our universe, we must look for homogeneous and isotropic solutions of Einstein's equation. Those solutions were found in the 1920's (I think) and are now called FLRW
WP solutions. There are three such solutions, and all of them describe an expanding universe. To be more specific, they all describe spacetimes that can be "sliced" up into 3-dimensional spaces that we can think of as representing "space, at time t" for different values of t, and in a specific mathematical sense, space at time t is "bigger" than space at time t'<t, and this size goes to zero as t goes to zero.
However, there's no event in a FLRW spacetime that has the time coordinate 0. Every event has t>0. The singularity isn't an event in spacetime. The "big bang" is just a name for the limit t goes to 0. So the phrase "when the universe was a singularity" doesn't quite make sense. The word "when" refers to a specific value of the time coordinate t, but the universe wasn't in any way "singular" at any t>0 and there is no t=0 in these solutions.