I watched Prometheus last night.

Making the assumption that aliens do exist. For what reasons are the crooks in power hiding the truth from us?

I think it's clear to see that hiding information like this is not in the public's best interests, so the "it's for our own good" theory just does not wash. I suppose the governments themselves could be terrified by the possibilities which could result from contact with aliens. The world being a better place? Overcoming poverty? Global corruption? War? Monopoly of power? The broken economy? Renewable energy? True world history? An abundance of knowledge? I guess the idea of an alien race floating down from the sky and answering all of these problems along the way, the idea probably terrifies the real power brokers.

Could the powers-that-be be in cahoots with the aliens? ie. they're harvesting/farming things from our planet? Who knows.

What are the main theories on this?

I truly believe that aliens do exist and that eventually we will be faced with undeniable evidence of the facts. What will happen after that point, your guess is as good as mine. The governments must have so much important information and evidence that they are hiding.

Maybe the aliens see us as a slave world, ie. we are all slaves to our governments. Maybe they have intentions of enslaving us for their own benefits. There's some sort of secret war going on between our governments and the aliens. That is pretty far out, but I guess it would make sense considering what Gary McKinnon has been saying about alien spacecraft and the top secret military operations in space.

What are the main theories behind world governments hiding the existence of aliens from the peasants?

You know, the question that is not asked often enough is:

Why would aliens care so much about hiding their existence to us? As technologically advanced beings, I am sure earth governments mean little to them.

So for the conspiracy to be true, both the aliens and all governments, HAVE to be hiding their existence. So why are they doing that? You tell me.

i'll post my theories.

1. aliens can't breathe our atmosphere, therefore can't land. they have no desire to contact earth, can't understand our language are curious and bold.

we are nothing to them. just another animal. they are scientific in nature and are here to monitor and study us.

2. the government monitors them, know they are not from earth and are powerless to stop them. why disclose that, have to listen to endless questions that they don't have an answer for, and a media that will blow everything out of proportion and cause the majority of the planet to go on the war path.

and possibly destabilize the status quo they have right now with ufo's.

3. the government captured a ufo and may or may not have made contact. roswell is as close to evidence you'll get to this theory.

thus advancing technology by 200 years in less than half a century.

The MAIN reason is because of the psychological impacts that would be created too many of us "not ready to be unplugged".

Maybe visting aliens enjoy Earth art, music, literature, sports, etc. and feel that open contact would spoil the show.

Power.

The government would lose power over the people. And we all know we can't have that!

If there are governments in cahoots, exchange, treaty agreement, trade relations, or any other sort of elbow rubbing with aliens (if such exist), it's a simple question of National Security.

If other nations know you're talking to aliens, they'll be jealous if they aren't and will either want in on the action as an ally, or potentially get all stabby with the missiles as an enemy.

Keeping such a thing secret keeps all of the allies from being a nuisance in begging like cry-babies to share, or else they'll pout and make lots of noise, and it keeps tactical advantage over enemies that are none the wiser over the technical tactical advantage.

Both parties understand the ramifications of what such a declaration would have on our society. And I'd also have to say that if the intention is to keep it a secret, both parties aren't doing a terrible job to be honest. Look for instance at the statistic that one third of Americans believe in UFO's, that's 100,000,000 People.

Personally, I believe both parties are drip feeding us bits and pieces of the secret slowly over time. In That UFO's intentionally fly low and illuminate their craft, or perform maneuvers in-flight that we can't do. And governments of the world intentionally soften us up by releasing documents showing that they have vested interests in the sightings of such craft. Or allowing Whistle Blowers to come forward and tell people their story.

The next step will raise the bar once again, maybe they'll be more incidents like the Phoenix Lights, or another Whistle blower will come forward.

I'm also pretty confident on the subject of the Curiosity Rover finding microbial life on Mars.... or rather that we already know that its there but we'll put on a song and dance for the public to make out like we all discovered it at the same time.

....Then more big sightings, Whistle Blowers of better and better credentials 'till eventually that 100,000,000 becomes 200,000,000 etc.... 'TIll we end up with "Ok, we admit it, we knew about it all along, but hey! You didn't need us to tell you that, right?!"