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The Universe


The Universe is all of space – including planets, stars, nebulas, galaxies, and all the apparently empty space in between them.

We don’t know whether our universe is limited (is there a boundary to it?) or whether it is infinite (it goes on forever.)

Even though the size of the entire Universe is unknown, we can at least measure the observable universe.

The Observable Universe

The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth at the present time. We can see this part of the universe because light from these objects has had time to reach Earth.

Observable universe

Illustration by S. Larson

There are at least 2 trillion galaxies in the observable universe – containing more stars than all the grains of sand on planet Earth.

Sombrero galaxy M104

A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)

The distance to the edge of the observable universe seems roughly the same in every direction. So the observable universe appears as a spherical volume – a ball – centered on the observer.

Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth.

galactic-map-boss-ruler Universes

Our entire visible universe is just one of these spheres. Observers very far away would have their own visible universe. Yet all of these separate visible universes are part of one larger universe.


Apparently mostly empty space

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays).

ESA (European Space agency) Kids: Story of The Universe

Universe has grown since the Big Bang

The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart.

ESA (European Space agency) Kids: Story of The Universe

What might exist outside the observable universe? 

We can’t observe beyond the boundaries of the observable universe because the light from beyond hasn’t had the time to reach us yet, in the 13.8 billion years since they first emitted light.

Beyond this? Nobody knows. The universe could potentially be infinite and have no boundary or it could end shortly after this observable region. Some physicists have estimated the size of the universe to be somewhere in the range of 200–250 billion light years. But we simply don’t know. Other scientists suggest that our universe is just one of an infinite number of multiverses.

– Jaime Sánchez, studied Earth & Planetary Sciences & Astrophysics, Quora: What might exist outside the observable universe?

It is vey difficult to comprehend or hypothesize what could be beyond a possible boundary of the universe. But we can use logic to consider possible ideas – See Four types of multiverses

Infographic: The Universe

From National Geographic

Click on this image to embiggen

Infographic the universe



Learning Standards


HS-ESS1-2. Describe the astronomical evidence for the Big Bang theory, including the red shift of light from the motion of distant galaxies as an indication that the universe is currently expanding,

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. The universe began with a period of extreme and rapid expansion known as the Big Bang. Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe.

Benchmarks: American Association for the Advancement of Science

The sun is many thousands of times closer to the earth than any other star. Light from the sun takes a few minutes to reach the earth, but light from the next nearest star takes a few years to arrive. The trip to that star would take the fastest rocket thousands of years. 4A/M2abc

Some distant galaxies are so far away that their light takes several billion years to reach the earth. People on earth, therefore, see them as they were that long ago in the past. 4A/M2de


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