Beyond snapping extraordinary pictures of faraway nebulas, the revolutionary Hubble Space Telescope has completely transformed our view of the universe since it was launched in 1990. By capturing the clearest, deepest images of the cosmos ever, Hubble has shed light on some long-standing mysteries perplexing scientists?while uncovering far deeper ones that have yet to be solved.

The push of dark energy

The universe was born in the Big Bang, but before Hubble, its age was uncertain. By measuring where distant galaxies are more accurately than ever before and how fast they are moving as the universe expands, the orbiting telescope resolved the cosmos is roughly 13.7 billion years old.

However, at the same time, Hubble and various ground telescopes unexpectedly discovered that the universe's expansion is accelerating, instead of slowing down as one might expect due to the pull of gravity from galaxies. The repulsive force driving this accelerating expansion, dubbed dark energy, which makes up some 70 percent of the universe, remains "one of the greatest mysteries in science," said Hubble senior project scientist David Leckrone.

Dark energy has prompted new theories regarding the origin of the universe, such as one where clashing membranes of reality trigger endless cycles of cosmic death and rebirth, as well as the fate of the universe, raising the possibility that dark energy ends the universe in a Big Rip. Future progress on understanding dark energy's nature will likely require a dedicated dark energy space mission, "for sometime in the middle of the next decade, perhaps," Leckrone said.

The pull of dark matter

Galaxies don't have enough normal matter to hold together the giant clusters of galaxies they are in, leading scientists to speculate on the existence of gravity from unseen "dark matter" pulling to keep galaxies together.

Although what dark matter actually is remains a mystery, Hubble did help show how much there is of it out there, by looking for how much dark matter's gravity warps space-time and thus distorts light from distant galaxies. The space telescope helped reveal there is some five or six times more dark matter than normal matter in the universe.

Hubble also has, with other telescopes, developed the first 3-D map of dark matter. "This helped show the clumpiness of dark matter has apparently increased over time, showing it exhibits ordinary gravity, as opposed to something else," Leckrone said. Better understanding how dark matter behaves could help scientists better understand what it actually is, he added. In the meantime, the 3-D dark matter map helps explain "how the universe acquired the large-scale 'web-like' structure that we observe in the pattern in which galaxies are distributed over the sky," he added.

Other mysteries solved and unsolved

The biggest explosions in the universe

Satellites first discovered gamma ray bursts, the biggest explosions in the universe, in the late 1960s, but scientists had little idea where they came from. Hubble helped discover they originated for the most part come from hot, young, very massive stars in distant galaxies, "which we think catastrophically collapsed on themselves to produce these gamma ray bursts," Leckrone said. "We believe they must be going off all over the place out across the universe."

Photographing worlds

Neither Hubble nor any other telescope so far has directly imaged an exoplanet. "There was one object that was claimed to be a planet, and it may well be, but it's a huge distance from its central star and is abnormally bright, so I don't think it really is," Leckrone said.

Protoplanetary disks

Hubble was the first to directly image the disks of gas and dust where planets are born with great detail, pictures sharp enough to reveal the gaps in the disks that nascent worlds carved out as they orbited their stars. "These findings help shed light on how worlds form, although there's quite a ways to go with that research," Leckrone said.

Hubble's successors

The James Webb Space Telescope is intended as a significant improvement over Hubble, an orbital infrared observatory capable of picturing substantially fainter objects. It might even "have a fighting chance to directly image a planet around another star," Leckrone said.

As light shed in earlier ages of the universe naturally shifts toward the infrared, distorted as it is by the expansion of the universe, Leckrone added the James Webb Space Telescope should also help peer back to an era unseen until now, when the first galaxies formed back when the universe was only a few hundred million years old. Investigating these early galaxies could shed light on the poorly understood process of how galaxies like our own Milky Way formed.

Still, Hubble will continue to prove invaluable for imaging the universe at visible and ultraviolet wavelengths, Leckrone added.

"Although the James Webb Space Telescope can be viewed as a successor to Hubble, it is not a replacement for Hubble," Leckrone said. "Ultimately, a much larger ultraviolet-visible telescope in space will be needed to carry on Hubble?s work."