Question

Pls explain the universe expanding theory briefly?

Answer 1

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang ...

Answer 2

there is a direct relationship between the speeds of distant galaxies and their distances from Earth. these galaxies are moving away and hence universe is expanding.