Dark matter is a form of matter that affects the "normal" matter (i.e. gas, stars, bicycles, etc.) only through gravity. That is, it doesn't collide with the normal matter, and it doesn't emit light.
Because it doesn't shine, we can't see it directly. When we nevertheless are pretty sure it exists, it's because there are many other ways to infer its existence, and even though these methods are very different and mutually independent, they give the same result: There is roughly 5.7 times as much dark matter as normal matter.
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