I used to think aliens exist or have visited us. Now? I'd say no.

The way the Universe works, aliens would have to be pretty close to us, or travel at the speed of light (faster than the speed of light in all likelihood, though, via warping space time), or travel through worm holes.

Our understanding of physics isn't perfect but, from what we do know, a lot of that is pretty much impossible. On top of all that, the Universe is very hostile towards life, so even traveling through space can/will get you killed.

There's quite a few reasons for the Fermi paradox, so I'm at:

• The visible universe is a hologram and that's why we can't see evidence (light) of alien civilizations
• We're very early / the first form of life
• They exist outside of the visible universe
• Rare Life/Earth
• Great Filter
• We're living in a simulation