The major visible components of galaxies are stars, with the space between stars being filled with interstellar ("between stars") medium (ISM). ISM is basically gas and dust. The gas of ISM is mostly hydrogen and helium, and the dust is small bits of carbon, silicon, and oxygen.
Gravity has arranged stars, gas, and dust into various structures within galaxies, such as nebulae, planets, and black holes (and nearly all large galaxies contain a supermassive black hole at the center). In addition, roughly 90% of the mass of galaxies is the mysterious "dark matter", which can't be seen but is assumed to exist because it exerts a detectable gravitational influence. All these pieces are held together by gravity into what we call galaxies.
About 90% of the mass in most galaxies seems to be dark matter - mysterious material that exerts gravitational force, but aside from that we know basically nothing about it, since it doesn't interact with other matter except by gravity.
The remaining 10% are stars, nebulae, and the like.
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