Welcome to the Universe (And Its Giant Hidden Parts!)

Chapter 1: Welcome to the Universe (And Its Giant Hidden Parts!)

The Big Question Nobody Could Answer

Imagine you walk into the biggest, sparkliest toy store in the world. The aisles stretch out farther than you can see. There are glowing things on the ceilings, spinning things on the walls, glittery things in jars, and giant bouncy-ball planets hanging from above. A friendly shopkeeper walks up to you and says, "Welcome! Pick anything you want. But first, let me tell you a secret about this store."

"Only the things in these display cases—everything you can see, touch, hold, and weigh—make up about 5% of what we sell. The other 95% is invisible."

You'd probably blink. "Invisible how? Like, hidden behind the wall?"

"Not exactly," the shopkeeper says, smiling. "The 95% is *right here*. It takes up space. It has weight. It pushes and pulls on everything. But if you close your eyes and try to grab it, your hand goes right through. We only know it's there because of what it *does* to the things we *can* see."

Sound crazy? That's basically what scientists have discovered about the real universe. The galaxies, stars, planets, moons, comets, asteroids, dust clouds, and glowing gas that you've seen in books and posters? All of that—every single thing your eyes can spot—adds up to only about five parts out of a hundred of everything that's out there. The rest, the other ninety-five parts, is made of stuff that doesn't shine, doesn't glow, doesn't reflect light, and most of the time doesn't even show up on our best cameras.

This is the biggest mystery in all of science. And you're about to start solving it.

This chapter is your welcome mat. We're going to take a quick tour of the universe, peek at what it's made of, and meet the two invisible giants that scientists have spent decades trying to understand: dark matter and dark energy. By the end, you'll be able to do something pretty cool—you'll be able to tell a friend the "cosmic recipe" of the universe (5% / 27% / 68%) and explain why "dark" doesn't mean "scary black hole" but instead means "we honestly don't know what it is yet."

Are you ready? Buckle your seatbelt. We're going big.

A Cosmic Pizza Party: How Much Stuff Is in the Universe?

Let's start with a pizza. A really, really big pizza—the biggest pizza you can imagine. Now let's pretend that this pizza is the entire universe. Every atom, every star, every drop of space, every bit of energy. The whole thing.

If we slice this cosmic pizza into 100 equal pieces, here's the shocking truth:

• **5 slices** would be everything we can see—every star, every planet, every galaxy, every moon, every comet, and even you.
• **27 slices** would be something called **dark matter**. It's real, it has gravity, but we can't see it.
• **68 slices** would be something called **dark energy**. It's even weirder—it doesn't even act like normal stuff. It acts like a stretching force.

This is the cosmic recipe: 5 + 27 + 68 = 100%.

When scientists first figured this out, they were stunned. Imagine baking a cake and realizing that 95% of what you baked is something you never put in the bowl. Where did it come from? What is it? How can you bake with ingredients you can't see?

Let's slow down and look at each part of the recipe in turn, because each slice tells a different story.

The 5% You Already Know

The part you CAN see is what scientists call ordinary matter or sometimes baryonic matter (that's a fancy word for "the kind of stuff you and your lunch are made of"). Ordinary matter is anything made of atoms. Atoms are the tiny LEGO blocks that build everything around you: your body, your dog, the air, the water in your bathtub, the chair you're sitting on, the screen you're reading this on, the stars in the sky, and the Earth itself.

Here's a quick atom refresher, just in case: inside every atom there's a center called a nucleus, made of particles called protons and neutrons. Whizzing around that nucleus are even tinier particles called electrons. Different atoms have different numbers of these particles, which is what makes a hydrogen atom different from an oxygen atom, which is different from a carbon atom, and so on. Every single thing you can touch, see, or smell is made of these tiny, tiny, tiny building blocks.

The 5% slice of our cosmic pizza includes:

• All the stars (like our Sun and the billions of other stars in our galaxy, the Milky Way).
• All the planets (eight in our solar system, and we now know there are billions of others orbiting other stars).
• All the moons, asteroids, comets, and meteors.
• All the gas and dust floating between stars.
• All the galaxies (groups of billions of stars held together by gravity—there are about two trillion galaxies in the part of the universe we can see).
• All the black holes (don't worry, we'll talk about those later—they're still made of "ordinary" stuff that has gotten incredibly squished together).

When you add up every galaxy, every star inside those galaxies, every planet around those stars, and all the gas and dust swirling around them, the total weight of all this ordinary matter comes out to about 5% of what the universe seems to be made of. That's it. Just five slices of our cosmic pizza.

You might be thinking: "But wait, the universe looks like it's FULL of stars! How can they only be 5%?"

This is a great question, and the answer is one of the most mind-bending things about space. The universe is mostly *empty*. Even in places that look "crowded" with stars—like the center of a galaxy or a giant gas cloud—most of the volume is still just... empty space. If you took a huge box the size of a building and put all the stars from a galaxy inside it, they would still be so spread out that you'd have to walk a long way to bump into one. The stars look close together in pictures, but the pictures are usually compressed, just like a city map squeezes miles of road into a small page.

So the 5% isn't a small amount in terms of *things*—it's a huge number of stars and planets. It's just a small amount in terms of *mass*, which is a fancy word scientists use to mean "how much stuff is in something." Mass is different from size: a tiny marble and a giant beach ball might take up different amounts of space, but the marble could weigh more if it's made of heavier stuff. When scientists say the universe is "mostly invisible," they mean mostly invisible in mass, not in count.

The 27% You Can't See (But Can Feel)

The next biggest slice of the cosmic pizza, at about 27%, is dark matter. The name "dark matter" sounds like something from a superhero movie, but it's actually just a placeholder name. Scientists called it "dark" not because it's black or spooky, but because they don't know what it is. The light from it doesn't reach us. It doesn't shine. It doesn't reflect. It doesn't glow. It just... isn't bright, doesn't give off light, and doesn't interact with light the way normal matter does.

But here's the twist: even though we can't see dark matter, we can *feel* it. How? Through gravity.

Gravity is the invisible pull that every piece of matter has. The Earth has gravity, which is why your shoe falls to the floor when you drop it. The Sun has gravity, which is why the Earth orbits around the Sun instead of flying off into space. You have gravity, too—technically, you are pulling on everything around you, just very, very, very weakly.

Now, when scientists looked at how galaxies spin, they noticed something strange. Galaxies rotate, kind of like a giant pinwheel made of stars. The stars closer to the center move faster than expected, and the stars far out at the edges move *much* faster than expected. In fact, the outer stars are moving so fast that, if the galaxy were made only of the stars and gas we can see, those outer stars should fly off into space like marbles spinning off a merry-go-round that's spinning too fast.

But they don't f