What comes to mind when you think of space? Imagine a friend boasting about a spacious building, stadium or museum they recently visited. Do you envision the building as vast and expansive? Is it simply very large, or does it seem empty?

The word spacious does not necessarily mean empty. It simply indicates that there is enough room to move around the objects inside it. Similarly, outer space is not completely empty. It is vast and forever expanding, but there's a lot of stuff in it.

As a kid, I used to wonder about questions like how far away the clouds are, what lies beyond Earth and how vast space is. This curiosity led me to pursue master's and Ph.D. degrees in astronomy. I have been teaching about these very concepts for two decades as a physics instructional professor at the University of Texas, Arlington.

## Where does space begin?

Earth is surrounded by different layers of various gases. At sea level, Earth's atmosphere contains about 100 billion molecules per cubic centimeter. As we ascend, the atmosphere becomes thinner and thinner.

At an altitude of about 50 to 62 miles (80 to 100 km), there isn't enough air for airplanes to fly. This boundary, which separates the Earth's atmosphere from outer space, is called the Kármán line, named after the Hungarian-American engineer and physicist Theodore von Kármán. Everything above the Kármán line is called space.

The density of space can vary, but, on average, it is around only 1 atom per cubic centimeter. Just imagine – a dice-sized cube of Earth's atmosphere contains billions of air particles. But in space, the same-sized cube contains only one or two particles.

## The interstellar medium and radiation

Space, or outer space, is a vast, near-perfect vacuum largely devoid of matter. This vacuum contains very few particles compared with Earth's atmosphere. However, it's not entirely empty.

Space is dotted with scattered matter called the interstellar medium, which includes hydrogen and helium atoms. These are the most common elements in space, and they exist in both charged and neutral forms. The interstellar medium also contains cosmic dust – tiny particles of various elements, including carbon and silicon, scattered throughout space.

High-energy particles called cosmic rays – which are primarily protons and the nuclei of atoms – travel through space at nearly the speed of light. Cosmic rays come from various stars including our Sun, as well as from supernovae, the material falling into black holes, colliding galaxies and more.

Space is filled with various forms of radiation, including cosmic microwave background radiation. This is remnant heat from the origin of the universe. High-energy cosmic events like supernovae and black holes also emit X-rays and gamma rays.

Magnetic fields generated by stars, planets and many other celestial bodies also permeate space. These fields influence the motion of charged particles by attracting or repelling them like magnets.

## Dark matter and dark energy

Scientists predict that an as-yet unseen form of matter that does not emit light or energy, called dark matter, makes up a significant portion of the universe's mass. Researchers guess that it exists because they can see its gravitational pull on other visible matter.

Similarly, scientists predict a mysterious form of energy called dark energy is driving the accelerated expansion of the universe. Unlike dark matter, dark energy is not related to matter or gravitational forces, but it's a property of space itself.

Imagine the universe as a balloon. Dark matter is like the balloon's material, affecting its shape, while dark energy is like the air being pumped into it. It doesn't change the balloon's material, but it influences how quickly the balloon expands.

## Does space warp? Can gravity bend space?

Space can also warp. Imagine that space is like a big, stretchy trampoline. If you put a heavy ball, like a bowling ball, in the middle of the trampoline, it makes a big dip downward. This dip is similar to how space warps around something big, like a planet or a star. The bigger the ball, the stronger the gravity and the deeper the dip.

If you roll a smaller marble across the trampoline with the bowling ball in the center of it, marbles might start going around the dip made by the bowling ball's gravity. The marble follows the curve of the dip, just like how planets follow the curve of space around the Sun.

Imagine that you shine a flashlight across the trampoline. If the light goes near the dip made by the bowling ball, it might bend a little bit as it travels. This is like how light bends when it passes near a very big object in space, such as a galaxy.

Think of a black hole, which has huge gravity, as an even bigger, deeper dip in the trampoline. If you rolled a marble too close to this super-deep dip, it would fall in and disappear, just like how things can get pulled into a black hole in space and can't escape.

So, space can warp or bend around big things that have lots of gravity, just like a trampoline bends when you put a heavy ball on it.

Space is more than just emptiness. It contains a mix of particles, radiation, magnetic fields and mysterious forms of matter and energy. Imagine space as a 3D playground, inside which objects like stars, planets, nebulas and galaxies exist and together make up our fascinating and complex universe.

Nilakshi Veerabathina, Professor of Physics Instruction, University of Texas at Arlington