This is a representation of the "APM" survey (APM stands for Automatic Plate Machine). There are well over one million galaxies in this survey – many more than there are pixels in this image. The gray scale is used to indicate the number of galaxies per pixel. Much has been learned about galaxy correlations from this survey, despite the fact that it is only two dimensional (redshifts were not taken). I few years from now, the Sloan Digital Sky Survey will have taken redshift measurements of about a million galaxies. (For more info follow this link)