Bars are large bodies of gas, dust, and stars. These structures are cigar shaped and are 2 times more long than they are wide. Bars rotate as a solid object. Scientists have learned through the use of infrared telescopes that the gases inside of these bars are moving very quickly. It is also thought that bars are just part of a stage in the life of spiral galaxies.
Bars are fairly common structures for galaxies to have.
This means that bars are formed easily. There are two theories as to why bars form in galaxies. The first idea is that they spontaneously through global disturbances in disc galaxies. Another theory is that bars are formed form interactions with near by galaxies. Both of these theories are based on numerical simulations.