Today’s Wonder of the Day was inspired by Ms. Keller's 3rd Grade Class from Greenwich , CT. Ms. Keller's 3rd Grade Class Wonders, “What is dark matter? ” Thanks for WONDERing with us, Ms. Keller's 3rd Grade Class!

Isn't scientific study great? Thanks to inquisitive minds throughout history, we know that gravity causes a ball to fall to the ground when we release it from our hands. We've developed vaccines to eradicate terrible diseases. We've created technology that improves the lives of people. Scientific study has also taught us a lot about exactly what makes up the stuff around us.

For example, we know that human beings are carbon-based life forms. We know there's oxygen in the air we breathe. We know the water we drink is made up of hydrogen and oxygen.

Science has given us a peek inside the atoms that make up all the matter around us. We know that protons, neutrons, and electrons make up the atoms of the elements that are the building blocks of all the matter we can see: people, things, and even the planets and stars in our solar system, galaxy, and the larger universe.

Yet science also gives us an idea of what we don't yet know. For example, what makes up the vast expanses of our universe? For many years, scientists thought most of the universe was empty space and all the other "stuff" was ordinary matter (also known as baryonic matter) like the things we can see on our planet and throughout our solar system.

As early as the 1950s, scientists began to suspect the existence of something massive in the universe that couldn't be seen. While studying faraway spiral galaxies, scientists calculated the mass of large objects by studying their motion. They expected to see objects in the center of these galaxies moving faster than objects at the outer edges.

What they found, though, was that material in the center and the outer edges traveled at the same speed. This told scientists that these galaxies had to contain more mass than they could see. This unknown, unseen source of mass became known as dark matter.

To date, scientists have not "seen" dark matter. Yet its existence seems certain based upon its gravitational effects that scientists can observe in distant galaxies. For example, scientists studying elliptical galaxies have concluded that these clusters of galaxies would fly apart if their only mass came from the objects that could actually be seen.

So what is this theoretical dark matter? Scientists only have hypotheses at this point. Some scientists believe it could be made up of ordinary baryonic matter that's just extremely hard to detect with our current scientific instruments. It could take the form of things such as dim brown dwarfs, white dwarfs, neutrino stars, or even supermassive black holes.

Other scientists believe dark matter is something else entirely. For example, some have theorized that dark matter might consist of non-baryonic particles such as WIMPs (weakly interacting massive particles), neutralinos, axions, or photinos — all of which are hypothetical particles that have never actually been seen.

Finally, some scientists believe that it's perhaps our current understanding of gravity that may need rethinking. Since the existence of dark matter stems from our observations of its theoretical gravitational effects on distant galaxies, could the laws of gravity that apply here on Earth somehow work differently in distant, large galaxies?

It will be interesting to see what scientists uncover as they continue to study dark matter. Current scientific estimates indicate that dark matter could make up approximately 25% of the mass of the universe. Up to 70% of the universe could consist of a force that theoretically repels gravity, known as dark energy, while ordinary matter — all the stuff we can see here on Earth and throughout our solar system, galaxy, and broader universe — makes up only about 5% of the mass of the universe!

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