Strictly speaking, when z < 0, this quantity is called a blueshift, rather than
a redshift. However, the vast majority of galaxies have z > 0.
The fact that the light from galaxies is generally redshifted to longer
wavelengths, rather than blueshifted to shorter wavelengths, was not known
until the twentieth century. In 1912, Vesto Slipher, at the Lowell Observatory,
measured the shift in wavelength of the light from M31; this galaxy, as it
turns out, is one of the few which exhibits a blueshift. By 1925, Slipher
had measured the shifts in the spectral lines for approximately 40 galaxies,
finding that they were nearly all redshifted; the exceptions were all nearby
galaxies within the Local Group.