Millions of galaxies mapped in their relative positions to each other show the large-scale structure of the Universe.

In 1929 Edwin Hubble estimated the value of the expansion factor of the Universe, now called Hubble's Constant, to be about 500 km/sec/Mpc. Today the precise value remains uncertain but is generally believed to be in the range of 45 to 90 km/sec/Mpc.

The value is important to cosmologists because it can be used to estimate the age of the Universe.

Results (February 2003) from the Wilkinson Microwave Anisotropy Probe (WMAP) indicate that the Universe is 13.7 billion years old   (H_o = 73 km/sec/Mpc).