When giants roamed the earth
This is always one of the first ideas to come up in a conversation about the dolmens. Giants made the dolmens, they are big and strong enough to work with blocks the wight of a typical car. Giants would easily cut the stones, transport them, shape them and assemble the structure.
Many ancient texts and some physical evidence support the idea of giants existing on the face of this planet in the past. I was surprised to find a fossilized bone belonging to a giant in a personal museum built buy a bloke named Sergey. (more about this in "Sergey's Museum").
The main problem with this theory is that it only tempts to explain one aspect of the enigma, how were the dolmen built? Especially under the pretext of giants, the most important question is "what for?". What is a dolmen to a giant? What use does a giant have for such a structure?