What does liberal economic theory say about monopolies?
Monopoly and competition are one of the central topics in microeconomics, so there many theories on the source of monopoly, and they are not universally applicable.  Government intervention is certainly one possible source of monopoly, but there are many others.

I don't think I could give a general answer that does the question justice, so I will just give links to some of the topics are related:


Cournot competition
Economies of scale
Network effect
Diamond search paradox
Monopolistic competition
Market contestability


The general area is known as industrial organization.  The classic text is Jean Tirole's "The Theory of Industrial Organization", though I think this only covers a small fraction of the topic.