Despite a lot of scientific evidence & many theories why does the origin of the universe remain a question to scientists

It's because, they have many theories that they can't really test. We don't know if a bunch of minerals gatherd around to make our planets or our planets could've been here for way longer then what we think. Or our planets could've been made by undintified life forms, we're not quite sure what it could be. So we made a conclusion, and it is that Gasses, and minerals formed our planets.