Matter is anything that takes up space. The smallest unit of matter, in which matter can no longer be divided, is called an “atom.” While atoms are way too small to see with the naked eye, they can be seen with the help of special microscopes. There is also matter that we can’t observe at all — even with microscopes. Scientists call this “dark matter.”

If we can’t see dark matter, how do we know it exists? 

According to NASA, when the universe began with the Big Bang around 13 billion years ago, stars slowly formed and gathered together into galaxies, which formed into clusters. Not only are these clusters made up of galaxies, but they’re also made up of all the stuff between them. 

Using x rays and gamma rays, scientists were able to detect extremely hot gases in these clusters. They measured how much gas there was between the galaxies within the clusters, and realized there’s five times the amount of material in the clusters than could be detected. The scientists also realized that clusters have a lot more mass than can be detected with observable light. This is why they know dark matter exists.

What is dark matter?

While we know dark matter exists, what it is, exactly, remains one of the greatest mysteries of our universe. According to the Dark Matter Day website, some scientists think dark matter isn’t matter at all, meaning it may contain particles we don’t know about. Or it could simply mean that the laws of physics are not yet complete enough to give us the whole picture of our universe. In other words, our understanding is still quite limited. 

Whatever dark matter is, scientists think it represents about a quarter of the universe’s total mass and energy. They also believe dark matter is the driving force behind the universe getting bigger and bigger, which they call “dark energy.” 

What is Dark Matter Day?

Dark Matter Day, 31 October, is a day that celebrates the quest to discover. If you’d like to join or plan an event, check out the Dark Matter Day website

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