Dark Nebulae Sometimes known as absorption nebulae these are dark concentrations of gas and dust which often have stars forming inside them. Dark nebulae can be seen if they obscure part of a bright nebula (eg. the Horsehead nebula) or if they block out background stars (eg. the Coalsack). The Horsehead Nebula Here is a map showing many of the dark nebula regions within 2000 light years plotted ...