Mysteriously, all of the distant galaxies show a red shift in their light,
and the farther a galaxy is from us, the greater its red shift is.
The only explanation we have now for a red shift is that the source
of the light is moving away from us.
So the observations of the red shifts of distant galaxies suggest that
the universe is expanding.
If somebody discovers something else tomorrow that can shift the
observed wavelength of light toward the red, then the whole idea of
the big bang and the expanding universe will be out the window.
