If you look back 15-20 years experts said 'other life in the universe is impossible' this was because it was theorized that our planets' composition was impossibly rare and so was the evolution that took place.

Now 15-20 years on with technology far more advanced, weekly they're finding planets which they believe to be almost identical to ours that could support life, some of which are older than ours. So I definitely think Aliens exist, whether they are 'among us' or can visit as they please........ (considering the distance / time it would take to travel here) that is another opinion and topic all together