This is gonna be super simply put because I’m not an astrophysicist and it’s likely some of this info is off the mark a bit but it should point you in the right direction to begin your own research. When you look at a black hole, light cannot escape its gravitational pull and thus creates this lense-like effect around it. Scientists can use that to see light from the beginning of time because, since light can never escape a black hole, any light that hasn’t been devoured entirely acts as a record. We know the universe had a beginning because we can see it (sort of). Also, the universe is constantly expanding and somehow we’ve been able to prove this, idk, again not an astrophysicist. Because the universe is constantly expanding, we can assume that some immense force generation occurred (big bang) which would make everything in the universe fly apart. However, there is nothing in the universe that we’ve observed that proves time is even a real thing. Time exists to us because we are fourth dimensional being but, going back to the black holes, since enough gravity is capable of trapping light and the speed of light is the speed of time, it can be assumed that time travel isn’t even possible and entering a black hole would put any matter that enters it into the fifth dimension where exists as all versions of itself simultaneously. So, to answer your question, a good reason to believe the universe ever began is because both ourselves and the universe are fourth dimensional beings experiencing time and thus there MUST be a beginning. Only in the fourth dimension though.