As science progresses in the quest to map the stars, the galaxies and all of the knowable universe, it appears that one aspect of "crackpot science" can be discarded. It is not a crackpot idea to believe that aliens could exist in myriad planetary systems in countless numbers.
Fermi's Paradox still applies, such that "Where are they?" is still a vexing question.
Some people on this planet assume that other lifeforms have been here already. If so, and I cannot prove that they have not, they are very good at keeping the evidence confusing. The work of governments cannot uncover their intentions, nor can theologians nor anyone at all. Notwithstanding the many books and huge number of believers, the evidence is always such that I, myself, am never able to see it.
I would not doubt that aliens exist out there in the billions of locations able to support life. What fraction of those locations actually do support life, I have no idea. What fraction of that life is able to evolve to a point where it could relate to us as we ponder the universe, I have no idea. Yet it is not a crackpot idea at all.
I think the main answer to "Where are they?" is: They are trying like heck to get places all around them, but the energy it takes is just too expensive. There is plenty of material, all sorts of atoms and energy all around their home star systems that take away any need to blast off for forsaken places like Earth to forage. Unless they can travel for thousands of years, frozen in stasis, regenerating as breeders on generation ships, or using technology beyond the most bizarre imaginations of scientists on Earth -- then there is just no cheap way to get here.
Imagine that you are allowed to vote for a tax -- the proceeds of the tax will allow you and your family to be blasted off into space for some location 2000 light years away, and it will use the greatest technology we have and only take 20,000 years to get there. Are you going to pay that tax? Even if it would only take 2000 years you would say no. That is a crackpot idea, and you will not pay. Who would trust the government to make star ships capable of lasting that long when we can't even keep our closest space craft in orbit more than a couple decades without massively expensive upkeep. No one will be able to service some star ship that is half way to a place 2000 light years away.
It is unlikely that aliens will think much differently about time and space than we do. Conjecture that they could jump across dimensions, shortcutting the time problem, is only a that -- a guess. We can think of no way to cross dimensions even with quarks or leptons or what have you, even for a second, let alone an entire star ship full of passengers.
Of course, given that some alien culture could be far beyond us in technology, perhaps they can think of ways that we are unaware of yet. Perhaps there are jumps and warps in space-time. Perhaps time is just an illusion that humans use for processing the information in our environment. Yes, but this, too, is just a conjecture. And the lonely prospect that, so far, no tourism cruises from other planets ever stops by Earth is a clue -- that maybe it is not so easy, regardless of how long a civilization might last.
Of course I have no proof that aliens are non-existent. That is not a scientifically valid statement, anyway. It would only possible to disprove a theory that aliens exist if it were possible to visit each and every location in space and time and then take a look. But the same could be said for Red Smurfs. I could look for Red Smurfs until I am blue in the face (ahem) and never know if they exist or not.