Here's the article.

I guess that's a reasonable answer. The author is a physicist, so I reckon he knows more about physics than I, who am not, possibly could.

If you don't wanna read the article, the TL;DR on it is: "Gravitons, precisely controlled by the hammer itself."

The only thing I'd have to add to this theory is: This could also explain why the hammer can hit so hard, far in excess of its observable mass.

What it doesn't explain is why it doesn't always hit hard enough to turn enemies into a pale pink mist. Maybe the dwarves who forged it have a failsafe so Thor doesn't just go around killing everyone, particularly those who may be worthy (i.e., Cap)?