Let's be honest. The universe is HUGE. It's so big my feeble imagination can't imagine it. There's bound to be some other form of life other than us, right? Hopefully one day we will find out the truth, and when/if we do, I'm sure it will be a big milestone in human history.

I believe there is a very good possibility that there are Extraterrestrials. I kinda doubt they have been to Earth tho.

I firmly believe there is some kind of life out there in space, judging by the sheer number of stars in our galaxy, and galaxies in the universe. The thing is we probably wouldn't even recognize it as life if we saw it.

Terrestrial animals all have common features because they have common ancestors somewhere in their family tree. Alien life has no relations to them, and it evolved in completely different environment, meaning that we should forget about heads, hands, teeth, lungs and brains, and instead we should accept that we literally won't be able to imagine an actual alien until we see one ourselves. No Xenomorphs, no Klingons, not even alien slug-anemones, just something beyond our imagination, like the color beyond the visible spectrum.

Something that irks me about sci-fi media is using terms like "alien animal" or "alien plant" or even worse "mammalian alien". Those groups came to be after million of years of evolution on Earth. There are many groups with diverse cell structure on Earth. Viruses don't even have cells, and they originated from Earth.
Why would aliens have exactly the same cell structure as us? They don't need to be composed of cells as we understand them, they don't need to contain DNA or any carbon based (organic) compound. I won't get so far as to say they aren't made out of matter (although dark matter aliens are an interesting concept, and antimatter beings would make sense, just not in our galaxy/universe). I'm just saying the rules life out there may be completely different.

What is not different is physics. While we wouldn't be able to comprehend alien technology at first, it wouldn't be anything humanity wouldn't be able to achieve (unless it requires some extremely scarce resources). Aliens still can't travel faster than light without messing with space time, and can't create energy from nothing. Spaceships would actually be much simpler than their crew, which we wouldn't be able to communicate with anyway.

But I still don't believe aliens have ever visited Earth. We would either know for sure, or we'd be all dead and the Earth cracked open and picked clean of minerals.