Dark Matter

The Horse Head nebula in Orion gets its shape from dark unilluminated gas silhouetting a bright illuminated part of the nebula. Clearly, if no stars are near a particular nebula it can not shine. This is true of the coal-sack nebula in the constellation of the Southern Cross.

Stars that have died a slow death cease to shine and can not be seen. Brown dwarfs are on the verge of being called dark matter.

All such matter is termed dark matter. It is obviously difficult to detect but although they do not shine, they still have mass and therefore have a gravitational attraction on nearby bodies. However, dark matter outside a galaxy is practically impossible to find. It is important to know how much dark matter there is in the universe because above a critical mass it would be powerful enough to stop the expansion of the universe.

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