if its observable that the universe is constantly expanding, then its logical that if you were to be going reverse in time the universe will begin shrinking but you don't know when it would stop shrinking or what caused it to start expanding, the farther you look the older light is that you see so your seeing an old universe if you look out into the universe far enough you see the cosmic microwave background, since space is literally empty nothing but that giant mass in all directions you look is a bunch of stuff with energy, it means you would need tons of matter concentrated together, and since this is an older view universe it would be completely within reason to conclude that you are viewing the universe that hadn't expanded anywhere near the size it is now so everything in the universe is super concentrated and close together. meaning the universe was all in a small spot and expanded in all directions so its pretty much an explosion.