We as a race tend to evaluate our surroundings on science as we conceive it with our limited natural abilities. The problem with that is this limits our views of our surroundings, never approaching the world as we should with an open mind. I have spent much of my life in the research and development world seeking new ways to do such things as track and identify an aircraft. I have seen things that have made me believe that for some reason we are being visited, it could be because of our natural war like ways. That we are the prisoners destined to remain here till we are rehabilitated and deemed worthy to rejoin civilization. These beings have no desire to harm us because we are like something to be studied, humored, and learned from! Just as we do of lower life forms on this third rock from the sun. I do believe if we ever become a threat we will be dealt with severally. (just as we deal with such things as fire ants)..Long story short we just in the last couple of years been able to move an object from one point to another in a lab without physical constraints. So please learn to keep an open mind! Never say no...

We find it hard to believe because we have more than a passing understanding of the sheet scale of space as well as an understanding of how unlikely and improbable harnessing completely hypothetical technology like worm holes and warp travel are. Seriously, they're both basically just magic - you'd have an easier time turning lead into gold or inventing cold fusion

I blame movies and TV series (Star Trek, Star Wars, Doctor Who and numerous others) for giving the impression that zipping around the universe is really fairly straightforward — just a few more steps in technological advancement and we'd be there. I don't think so. And neither would the aliens.

I believe aliens are out there. I do not believe they have visited in modern times, and probably not ever. Distances in space are unimaginable. The closest star would take over 50,000 years of travel to get there, and that's only 4.5 light years. Most stars in the Milky Way are thousands of times further. Life leaves footprints of sorts, for example an oxygen atmosphere. An advanced civilization would have radio and TV, which broadcast signals throughout space. SETI has found no signals from nearby stars. There are no advanced civilizations within a 100 light years (about 1,000,000 years or less travel time).

A nanoscale UFO can be pushed much faster, assume 100x speed improvement. It would take only 500 years to travel to our nearest neighbor, and 10,000 to travel out to 100 light years. The Milky Way is 100,000 light years across, and our nearest major galaxy neighbor is millions of light years distant.

This is the reason scientists are skeptical of UFOs carrying aliens, ships are too slow. I agree. It is unlikely aliens have visited Earth.

The last part of your last paragraph seems to indicate that you like the idea that we are all going to be saved by the well meaning 'Space Brothers' of a far more advanced civilisation out there in the galaxy someplace. Well , don't get your hopes up. We are more likely to be very much on our own in a vast and mostly empty Cosmos which doesn't care if we exist or not. All we really have is each other. Now how's that working out for us ?

There are loads of UFOs. But that is exactly what they are - unidentified.

To date, I can find no convincing evidence that we have ever been visited by aliens.

Just because something is possible, doesn't make it probable. As yet, we have no evidence that life exists anywhere but here on earth. We also have no evidence that anything of any substance can travel faster than light, rendering travel between solar systems a very difficult thing to do.

So for now, I have no reason to believe that aliens or alien craft have ever visited earth.

Two things.

One; there are indeed an enormous number of galaxies in the observable universe (estimates vary, however, with some putting it "as low" as 200 billion - which is still a huge number, but much lower than two trillion. However, the distances any spacecraft would need to travel in order to visit Earth from them are frankly mind-boggling: if there is life out there, a large part of it may be entirely unable to reach us.

Two: You're right in saying that a number of people employed by the Air Force, including some who are trained to spot and evaluate the performance of aircraft, claim to have seen craft they interpret as being of potentially alien origin. But, many others in the same role have not, and believe that UFO sightings are hoaxes or mistakes.

I do believe that there is a very high chance of there being life elsewhere in the universe; whether or not it visits us is simply impossible to say.

It is not difficult to conclude life, even intelligent life, not only exists but is rather common in the universe. The ability to transit hundreds, thousands, or millions of light years however would be a monumental technological achievement, one which for me would have to be seen to be believed.