"Coal sack" sailors called the black spot, which could be seen against the background of the milky Way near the constellation of the southern cross. Clearly such a black spot in the sky and in the Northern hemisphere near the bright star Deneb and the constellation Cygnus.
For a long time people thought that the black spots in the milky Way is a hole and the openings in the thickness of the stars that form, and believed that it is in these places we can look into the gaping emptiness of the world space.
But these are not holes, but on the contrary - "curtains", which consist of very dense clouds of cosmic dust.
The origin of cosmic dust?