This composite image of the Medusa galaxy (also known as NGC 4194) shows X-ray data from NASA's Chandra X-ray Observatory in blue and optical light from the Hubble Space Telescope in orange. Located above the center of the galaxy and seen in the optical data, the "hair" of the Medusa -- made of snakes in the Greek myth -- is a tidal tail formed by a collision between galaxies. The bright X-ray source found towards the left side of Medusa's hair is a black hole.
Groups & Clusters of Galaxies
Just as astronomers were getting used to the idea of not knowing what dark matter is, they got a completely different surprise at the end of the 20th century. Instead of slowing down after the Big Bang, the expansion of the Universe was found to be accelerating. Astronomers quickly did what they always do when they come up with something mysterious: they gave it a name. Now, we call whatever it is pushing the Universe apart "dark energy", but the truth is no one knows what it is.
When you look up at the night sky, you see a lot of things glowing like stars, planets, and galaxies. So it might sound strange to hear that most of the Universe is actually dark. The truth is the protons, neutrons and electrons that make up everything we can see - and that means with every telescopes we've got -- accounts for only about 4% of the mass and energy of the Universe. The rest is dark and mysterious. More specifically, about 70% of the Universe is what is known as dark energy; about 26% is so-called dark matter.
A lot of kids (and maybe adults) are scared of the dark. Most of us get over it once we realize there isnâ€™t a monster hiding in the closet or under the bed. But then scientists announce another finding about dark matter and we all are back peering from under the sheets.
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