How do I know what? That it's the position of those who want to reduce regulation (that market can fix itself on its own)? Or how do I know that it doesn't work when some entity gets too much control and power and prevents such fixing?

One of the examples of what a complete monopoly is about, was USSR (it's kind of an anti-example, but that's exactly the point - a monopoly is the opposite of the free market). I.e. in that case the state had a complete monopoly over the market. Imagine a corporation like MS being the size of the country and controlling all markets altogether. That's what needs to be avoided. The logic is, that if the monopoly does happen, expecting it to fix itself with the "invisible hand of the market" is absurd. It's like expecting a dictator to depose oneself. So the antitrust law is built as a safety measure to prevent such thing from happening.