Most other galaxies are moving away from us. Light from these galaxies is shifted to longer (and this means redder) wavelengths - in other words, it is 'red-shifted'.

To calculate the redshift of a distant galaxy, the most accurate method is to observe the optical emission lines and measure the shift in wavelength. However, this process can be time consuming and is thus infeasible for large samples, so we only have spectrometry measurements for a small fraction of all galaxies that we can see.

Photometric measurements are easier to obtain (but less accurate). The goal here is to predict the spectroscopic redshift from their photometric colours by training on galaxies for which we have both measurements. This is how the redshift can be calculated for galaxies which have no spectrometric measurements.

Dataset Used: Flux magnitudes from the Sloan Digital Sky Survey (SDSS) catalogue.