Pyramids were built by humans. Simple as that. As to the mystery as how it was done who cares.....
I am not golly enough to believe we are the only intelligent life forms out there but I can't provide proof that there is and no one can provide proof that there aren't.
That about sums it up. Common sense prevails.
Simple and yet mysteriously unknown? Sounds like a contradiction. Given our knowledge of the technological and industrial capabilities of that ancient civilization, I'd say it's more than tantamount to an actual miracle. Not so clear cut in fact, but common sense nonetheless.
The premise for there being alien life on other planets is based on the theory that life on Earth resulted from a congruence of ultra-rare conditions, and not by any design. The extrapolation of that theory is that all forms of life on Earth are evolved from the resulting microbe(s), by virtue of countless random genetic mutations advantageous to survival, and the element of luck in natural selection. And so we say, if it could happen here, surely it could also have occurred on at least one of the trillions of planets in our universe, or even multi-verse! Law of Averages right?
One of the problems with that is, we don't actually know exactly how life originally came to be. The evidence of which is that we cannot take non-living matter and cause it to become a living entity. And so that remains an unproven theory. The theory of evolution itself, whilst widely accepted (not in small part due to it being incorporated into education curriculums across the world while still unproven), has had more than it's fair share of technical issues. So much so that continued adherence to it has caused it to be branded 'the religion of scientists', as work on the theory is based on the faith that it will eventually prove true.
What of the question that science does not or cannot ask based on its axioms? The elephant in the room if you will. I find that when supposing from atop the Jenga stack, it is much safer to knock it all down first and then begin at the beginning. The Big Bang that caused an automated process of incalculable complexity. What caused it, and more important still, where did all that energy/matter come from? Well, some people say, I don't know, some people say it doesn't matter, other people say God.
Let's examine these answers. 'I don't know' is an honest one, but really means you haven't put much thought into it, compared to let's say, sex.
'It doesn't matter' is typically a cop out. If this question does not matter, then nothing else does.
'God' begs the question, which or whose God? Yours or mine? The Jewish, Christian/Catholic, Muslim, Hindu, Buddhist, Taoist, Pantheist, etc.. God? This is problematic, as it places unacceptable restrictions on the investigation via pre-conceived notions of what should be, influencing the impartialness, mostly due to the tendency to try to adhere to doctrine.
So, regarding matter/energy, what are the possible ways it could have come into existence? Was it always there, or did it create itself, or was it created by another entity? There is no other possible consideration at this point, so let's go through these.
1. Has matter/energy always existed?
In short, no. The nature of it, is that it is always in motion, always subject to cause and effect. To say that energy/matter has always existed is to say that a chain of causality has occurred without ever being initially caused.
2. Did matter create itself?
Again no. That which is not, cannot. Simple enough.
3. Was it created by another entity (i.e. something not of matter)?
Now generally, it is poor logic to assert something as fact as a result of certain other assertions being untrue. The exception to this rule is the process of elimination. When you have eliminated all possibilities but one, then the last remaining possibility must in some way be true if the outcome is observable. Meaning to say, there must be a creator, as there is creation.
The most common issue or rebuff to this statement is the classic question: Who then created the creator? We speak of cause and effect, as all things in our universe, and the universe itself are subject to it indiscriminantly and without exception. We know this because it is observable through the properties of matter. However, and you might have guessed, we cannot by necessity apply this same restriction to the creator, as we have no verifiable witness to the properties of the creator (being not of matter), and so we cannot subject the creator to cause and effect as we do with matter, thus rendering the question invalid within the parameters.
So, after eliminating all that cannot be, we are left with what must be based on what undeniably is. That is, we exist, and so we must have come into existence, with the only possible explanation being that we were created by an entity that is not subject to cause and effect, but rather the initiator of it. To deny this is to deny your own existence (read: a pointless exercise).
Now knowing that the universe was created by design, could there be life on other planets? You're still no closer to knowing! 
Seriously though, there is far more to this than what I have divulged so far, and much more can be extrapolated still. Though if you were to ask me now if I believe there is alien life on other planets, based on what I have discovered I would say almost certainly not. Now, parallel instances of Earth with human and animal life, that I would say is far far more likely.
