The light year is NOT A THEORY, but a measure of cosmic distance, which is close to six trillion miles.
Light has been measured by precision instrumentation to travel close to 186,272 miles per second.
3.26 light years equals one parsec, which is the base unit for finding stellar distances through parallax, using a base measurement equal to the diameter of Earth's orbit around the Sun (186 million miles).
The light that defines the Andromeda galaxy (M31) to our eyes, left Andromeda 2.5 million years ago. What we see now is what M31 looked like 2.5 million years ago.
What's impossible about that?