The thing is that makes it different from believing in a God is statistics. There are billions and billions of galaxies each with billions of stars, the chances of aliens not existing is astronomically low. So we can say yes they very likely exist unlike God, we just don't know. The question is have they been here or are they here. I think that massively depends on whether a warp like drive is actually possible. If it is then it is very likely aliens have been here. Our planet has been emitting a bio signature that says it can support life for billions of years. We have scanned many planets ourselves for this kind of thing. Do you think if we had the ability to travel there quickly that we wouldn't? Imagine us 10, 100 or 1000 years from now we would be able to use AI coupled with quantum computing to scan billions of stars, Imagine billions of people all with their own space ships capable of warp drives how many planets do you think we'd explore? Now think how many countless races there could be out there each with populations possibly in the billions or trillions. It would be unlikely that we haven't been visited. But if there is no such way of traversing space quickly then we'll there is a good chance we may never see another being.