A light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). It's a unit of astronomical distance.

Millimeters are commonly used to measure small lengths in everyday life, such as the thickness of a coin or the size of a screw.

Because a light-year represents an incredibly vast cosmic distance, while a millimeter is a very tiny terrestrial measurement. The difference in scale is astronomical.

Yes, the distance to the Andromeda Galaxy, our nearest major galactic neighbor, is about 2.5 million light-years away.