The Zone of Avoidance (ZoA) is a region of sky that appears devoid of extragalactic objects at optical wavelengths. Covering roughly 20% of the sky and centred along the galactic equator, the ZoA is irregular in shape, varying in height above and below the Galactic plane with the widest point located towards the centre of our Galaxy in the constellation Sagittarius. Extragalactic objects do exist in this region of sky, but the extinction through the Galactic plane is very high, as dust and dense gas in our own Galaxy absorb the light from background sources. The high concentration of stars in the Galactic plane also make faint extragalactic objects difficult to detect within the ZoA.
To penetrate the ZoA, we must observe this region at wavelengths that are not as affected by dust. For late-type galaxies, far infrared surveys (such as IRAS) have reduced the ZoA to about 10% of the sky, while radio observations of neutral hydrogen clouds have detected many galaxies that the infrared surveys missed. Near-infrared and X-ray surveys have played a similar role in detecting early-type galaxies, and we now have a much better idea of the inventory of extragalactic objects located behind our Galaxy.