There's something strange happening in our galaxy: it's spinning fast enough for stars to fly away, but there's something that holds them together.
The substance that acts as a gravitational glue is dark matter. But it's incredibly mysterious: because it does not emit light, no one has ever seen it directly. And nobody knows what it is, though there are many wild hypotheses.
For our galaxy – and most others – to remain stable, physicists believe that there is much more dark matter in the universe than normal matter. But how much?
Recently, astronomers using the Hubble Space Telescope and the Gaia Star Chart of the European Space Agency (ESA) have attempted to weigh the mass of the entire Milky Way galaxy.
It's not an easy thing. For one thing, it's difficult to measure the mass of something we're in. The Milky Way measures about 258,000 light-years. (Remember, one light year equals 5.88 trillion miles ̵
Their answer: The galaxy weighs about 1.5 trillion solar masses , This number helps us to see in perspective how small we are.
For example, assume where stars fit in the Milky Way.
If you are lucky enough to get a completely dark, clear sky for stargazing then this is the case. It is possible to see up to 9,000 stars above you. So many are visible to the naked eye. There are another 100 billion stars (or more) only in our own Milky Way – and yet it is only 4 percent of all matter or matter in the galaxy.
Another 12 percent of the mass in the universe is made up of gas (planets, you, me, asteroids, all of which is a negligible mass in the galaxy's total balance). The remaining 84 percent of matter in the galaxy is dark matter, explains Laura Watkins, a research associate at the European Southern Observatory and a contributor to the project.
The scale of the galaxy and the extent of the galaxy puzzle of what it is made of is really hard to think through. So we've tried to visualize the scale of the galaxy and the scale of dark matter based on recent ESA Hubble findings.
As a visual metaphor we have built a mass tower. You'll see that all the stars in the galaxy are just a searchlight at the top of the building. Well, nobody knows what's going on in there.
The mass of the Milky Way, visualized
To make the mass of 1.5 trillion suns visible, we start small. That's the earth. It has a mass of 5,972 × 10 ^ 24 kg.
This is the Earth compared to the Sun. The sun is 333,000 times more massive than the earth.
Now let's imagine the mass of the 100 billion stars (or more) stars in the Milky Way.
Another 12 percent of the mass in the galaxy is only gas floating between stars (mainly hydrogen and helium).
This is what the gas looks like on this visual scale.
What about black holes? "It's a little harder to give an exact number of how much they contribute to the total mass, because we do not know how many there is, but it will be a very, very small fraction," Watkins explains. "The supermassive black hole at the center of the Milky Way has about 6 million solar masses," which is tiny on the scale of the galaxy's entire mass.
And it's tiny on the scale of the galaxy's most abundant, mysterious matter: the dark stuff. Again, 84 percent of the galaxy is dark matter.
Dark matter does not seem to interact with normal matter at all, and it is invisible. But our galaxy and our universe would fall apart without them.
Scientists abandoned their existence when they realized that galaxies are spinning too fast to be alone with the mass of stars. Think of a carnival ride that swirls people around. If it turned fast enough, those drivers would be torn from the ride.
The consideration of the "dark matter" and the resulting gravity made her galaxy models stable again. There is also other evidence of dark matter: it seems to produce the same gravitational lensing effect (meaning that the tissue of space-time is distorted) as with normal matter.
Let us now try to visualize the mass of dark matter in relation to the mass of the stars and of the gas.
And remember: this is just our galaxy. There are several hundred billion galaxies in the universe.
Also remember that dark matter is not the biggest puzzle in the universe in terms of size. About 27 percent of the universe is dark matter, and only 5 percent is the matter and energy that you and I see and interact with.
The remaining 68 percent of all matter and energy in the universe is dark energy (which accelerates the expansion of the universe). While dark matter holds individual galaxies together, dark energy drives all galaxies in the universe apart.
What you can see in the night sky may seem enormous: thousands of stars and solar systems that may be explored. But it's just a tiny bit of what's really out there.