Hubble is in space and the mirror is exposed, so how has the mirror stayed clean? Is it possible that space debris and micro particles could hit it?

The Hubble Space Telescope floats against the background of Earth.

NASA

The Hubble Space Telescope’s 7.9-foot-diameter (2.4 meters) primary mirror remains remarkably robust and resilient. Astronomers have not measured any degradation in the amount of light the telescope collects and sends to its onboard instruments.

That said, the mirror has probably undergone micrometeorite erosion, but there is no direct way to measure the amount of “pitting.”