Hubble Focus: Dark Universe

This e-book is part of a series called Hubble Focus, which presents some of Hubble's recent cosmic discoveries. Its cover, shown here, features the Abell S1063 galaxy cluster. A soft blue haze, called intracluster light, casts an otherworldly glow among innumerable galaxies. The stars producing this glow were thrown free of their home galaxies. These stars now live solitary lives, no longer part of a galaxy but aligning themselves with the gravity of the overall cluster. Astronomers using Hubble found that tracing intracluster light offers a good indicator of how invisible dark matter is distributed in the cluster.

NASA Goddard Space Flight Center

This NASA Hubble Space Telescope downloadable e-book is the fifth edition of the Hubble Focus series. Called “Hubble Focus: The Dark Universe,” the e-book highlights the mission’s recent discoveries about two mysterious components of our universe, dark energy — an unexplained cosmic pressure that’s speeding up the universe’s expansion — and dark matter, an invisible substance detectable only by seeing how it gravitationally influences visible matter.

This Hubble image features an unusually close-knit collection of five galaxies, called The Hickson Compact Group 40. Though such cozy groupings can be found in the heart of huge galaxy clusters, these galaxies are notably isolated in their own small patch of the universe. One possible explanation is that there's a lot of dark matter holding these galaxies together. Hubble’s sensitivity, resolution, and wavelength range help astronomers probe the fundamental nature of the universe, including its most mysterious components.

NASA, ESA, STScI; Image processing: Alyssa Pagan (STScI)

A Cosmic Ghost Hunt

The trillions of stars, planets, galaxies, and other visible objects strewn throughout the cosmos represent less than 5 percent of what’s truly out there. Visible matter is like the tip of an iceberg, or the foam on top of a latte. All the rest of the universe, dark matter and dark energy, is mired in mystery.

Dark matter is a phantom in the machinery of the universe. Though it makes up the vast majority of the universe’s bulk, dark matter would evade even the best “ghost hunters” because it’s invisible, detectable only through its effects on normal matter. Its gravitational pull is the muscle of the cosmos, holding together both individual galaxies and galaxy clusters. Although scientists have long seen evidence of its existence, dark matter’s true nature remains one of the biggest mysteries in modern physics.

Hubble’s cosmic detective work offers clues by exploring the way matter, both normal and dark, is structured and distributed throughout space. Some of the mission’s observations have even tested theories about the type of particle that could make up dark matter. But Hubble’s observations haven’t always matched predictions, hinting that our theoretical models still have several missing pieces.

Under Pressure

Shock waves of surprise rippled through the scientific community in 1998, when Hubble observations of supernovae in more distant galaxies helped show that the universe actually expanded more slowly in the past than it does today. That meant the expansion of the universe was not slowing down due to the attractive force of gravity, as many thought it should — it was speeding up.

Today, we still don’t know the exact cause of this mysterious acceleration, but theoretical cosmologists coined the term “dark energy” to describe it. Dark energy is so weak that gravity overpowers it on the scale of humans, planets, and even within the galaxy, which is why it was unobserved for so long.

Pinkish-purple filaments fill the screen. Areas where they are more dense are brighter and a bit yellow, while voids are black.

An artist’s impression of the cosmic web. Gravity builds a vast cobweb-like structure of filaments tying galaxies and galaxy clusters together along invisible bridges hundreds of millions of light-years long. Hubble studies the cosmic web to help us better understand the invisible forces that have shaped our universe into this structure.

Volker Springel (Max Planck Institute for Astrophysics) et al.

Dark energy is present in the room with you as you read, even within your body, but gravity is much stronger at smaller scales, which is why you don't fly out of your seat. It is only on an intergalactic scale that dark energy becomes noticeable — and since it’s everywhere, it even overwhelms the dark matter! Hubble has helped gather very precise measurements of the universe's expansion rate, but its findings underscore a nagging discrepancy. The universe is expanding faster now than was expected from its trajectory seen shortly after the big bang, and no one yet knows why.

The perplexity surrounding dark energy and dark matter indicates that for all we’ve learned about the universe, we still don’t know much about its underpinnings. Studying these mysteries opens the door to discovering exciting new physics.

“Hubble’s incredible scientific power continues to drive modern astronomy,” said Jennifer Wiseman, Hubble’s senior project scientist. “Dark matter and dark energy were not in mind when Hubble was first designed, and yet by detecting the impacts of these unseen cosmic phenomena, the Hubble Space Telescope is once again transforming our understanding of the universe.”

This e-book is compatible with most electronic devices. If you have problems with the EPUB version, we suggest trying the more widely accessible PDF version.