Personally, I have never seen a UFO let alone an alien; however, given the jaw dropping size of our galaxy, I do not discount the existence of aliens and whatever form they may take, it does not necessarily have to be anything that we are familiar with on earth.

I imagine that any aliens that would be capable of inter-galactic travel would probably have managed the creation of an energy source that is beyond our imagination and comprehension.

There is the possibility that alien life is already present on earth and may be so inconspicuous we do not even know of it’s existence .Humans were not even aware mites were living and reproducing at prodigious rates on their own faces until 1841.

Yes there only needs be one doomsday technology, whose dangers can not be understood by more primitive tech. waiting to be discovered, and everybody goes the same way.

When alien existence is pondered it is usually based on life that is only recognizable from a narrowly limited human frame of reference.Life may exist that is so removed and diverse from our perspective that it does not in any way conform to or require any parameters we could possibly imagine for its existence .Also with the theoretical possibility of the existence of Bubble universes (multiverse) that may each be comprised of a different law of physics this further opens up the possibilities of life forms so infinitely remote from ours it is not even fathomable .

I've had a big problem with the Fermi Paradox for a long time, even the "conservatives" assume that we can build a compact, efficient fusion reactor that can operate for decades at a minimum. I think fusion is likely but it will be optimized for deuterium/ tritium (the easiest route) and like other large power sources will be large stationary plants with limited service lives. Interstellar travel will always be so slow, difficult and resource expensive that anything beyond limited research probes will never be worthwhile.