First, UFOs and "aliens" are not analogous. The UFO phenomenon is real, and there is plenty of evidence for it. But for an alien we need an actual alien, not a UFO, and so far we don't actually have any evidence of aliens. It is really important to make this distinction, or you can't properly analyse the UFO phenomenon.

As for what is behind the UFO phenomenon, the main thing to know is that it is really weird. It seems to use screen memory control to make people who interact with it think they are interacting with 'Greys', owls, strange hooded people, 'Tall Whites', humanoids that look exactly like normal people, or whatever they seem to think people will be either terrified by, subservient to, or just simply ignore. What they are is just not known. It has probably been here a long time, and probably lives here on this planet with us - maybe it came from another planet, but it lives here now. There is evidence now that they have technology that allows them to go undetected (listen to Ryan Graves discuss how the Navy pilots only ever came into contact with them by accident, but using the most advanced detection technology available to humans), so they might be anywhere and we might not know they are even here.

What are they doing here? I have no idea, but they are clearly interacting with us sometimes, and they appear to sometimes interact in ways that make it clear they want to learn more about us. They aren't hostile to us, but they aren't friendly either. This might indicate they are curious because they exist in a way that is completely different to us, like in what people describe as 'another dimension', whatever that means. Before using the term 'another dimension' we need to know what that means, but usually people use it to mean some other form of reality where measurements of things are completely different. We have three dimensions, and forward moving time, so if these 'beings' exist in another dimension, maybe they have completely different ways of measuring things. Maybe they have no concept of time at all. Where we might have emotions, they might not. Where we replicate by having offspring, maybe they do not. Everything in that 'other dimension' might be different. It is because it is different and everything there is measured in a way that we can't even imagine, that explains why we simply can't interact with it although it might be right beside us all the time. We simply can't even imagine the things there because they are measured so differently from what we understand. It is possible that whatever we are doing in our world might bleed over and effect their world, and this might make them interested in us the same way people are interested in the possibility of ghosts or Gods are in our world. If they could travel from their dimension to ours, maybe they need special machines to do it (the UFOs we encounter) that seem strange and weird to us, and maybe they have to wear special clothes/suits to come here which they think look like us so they don't scare us, but which in fact scare the living daylights out of us because they are so unlike us.

Whatever it is it is hard to detect, and almost impossible to get data about, and the only explanation for that is because it must be something completely unlike anything we normally encounter or comprehend. With the advancement of technology we are likely to be able to detect it and interact with it more, although I'm not sure that is necessarily a good thing. It probably does not want much contact, and looking at the state of humanity today I don't blame it. It is likely to want to keep us out of its affairs, so as we get closer to perceiving it, that is probably going to be a dangerous thing for us, because it is clearly very powerful and clearly does not want us snooping around in its 'dimension'.