The Dark Horse Nebula or Great Dark Horse is a large dark nebula

Dark nebula

A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula or that it blocks out background stars . The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger...

, which as seen from Earth, obscures part of the upper central bulge of the Milky Way

Milky Way

The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

. The Dark Horse lies in the southern constellation

Constellation

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Ophiuchus

Ophiuchus

Ophiuchus is a large constellation located around the celestial equator. Its name is from the Greek "serpent-bearer", and it is commonly represented as a man grasping the snake that is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century...

 (the Serpent Bearer), near its borders with the more famous constellations Scorpius

Scorpius

Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...

 and Sagittarius

Sagittarius (constellation)

Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...

.

This region of dark nebulae is called Dark Horse because it resembles the side silhouette of a horse and appears dark as compared with the background glow of star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s and star cloud

Star cloud

A star cloud is a group of stars that appear to be in the same position in the sky. These are not true star clusters, but rather dependent on the point of view of the observer. The stars that make up the cloud do not actually reside close to each other, they are just in the same line-of-sight.-See...

s. It is also known as "Great" because it is one of the largest (in apparent size) groups of dark nebulae in the sky.

The rear of The Great Dark Horse (its rump and hind legs), is also known as the Pipe Nebula

Pipe Nebula

The Pipe Nebula is a dark nebula in the Ophiuchus constellation. It is a large but readily apparent pipe shaped dust lane that obscures the Milky Way star clouds behind it...

, which itself carries the designation B77, B78, and B59. (The 'B' numbers named after the astronomer Edward Emerson Barnard, who catalogued dark nebulae.) The Snake Nebula

Snake Nebula

The Snake Nebula is a dark nebula in the Ophiuchus constellation. It is a small but readily apparent S-shaped dust lane that snakes out in front of the Milky Way star clouds from the north-north-west edge of the bowl of the Pipe Nebula. Its thickness runs between 2′ and 3′ and runs...

 extends from the Dark Horse to the Rho Ophiuchus nebulosity.

The ability to see The Great Dark Horse with the naked eyes is an indication that the skies are very dark, i.e. not affected by (urban and industrial) light pollution

Light pollution

Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...

.