Question

find the word that matches the definition:
a phenomenon in which the wavelengths of light from a
source shifts to the red end of the spectrum

Explanation:

The definition describes a phenomenon where light wavelengths shift to the red end of the spectrum. 'Redshift' is the term for this phenomenon in physics (light - related concepts). 'Irregular galaxy' refers to a type of galaxy shape, 'theory' is a general explanatory concept, and 'galaxy' is a large collection of stars, gas, etc. So'redshift' fits the definition.

Answer:

redshift