The greatest insights often make the world seem like a larger place than it was before. In Hubble's case, the most important and perhaps most confounding discovery it helped find accomplished just that, by revealing the universe was expanding faster than anyone had known. 

NASA launched the Hubble Space Telescope, a joint effort by NASA and the European Space Agency, on April 24, 1990 aboard the space shuttle Discovery to much fanfare that soon fell flat. A flaw in the telescope's optics gave it blurry vision and turned the iconic space telescope into a potential boondoggle in orbit.

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But Hubble was built to be upgraded by astronauts riding NASA shuttles. In 1993, the first crew of space mechanics fixed the Hubble telescope's vision flaw, with four more maintenance and repair missions to follow.

NASA's last trip to Hubble was in May 2009, when the crew of shuttle Atlantis paid one final service call to the orbital observatory. They replaced Hubble's old batteries and worn out parts, revived broken cameras never designed to be fixed in space and added two new instruments. The result: A Hubble Space Telescope more powerful than ever.

Here's a look at some of Hubble's greatest astronomical achievements:

Hubble's greatest discovery

Scientists have dubbed the suspected culprit behind this accelerated expansion "dark energy," and it is now thought to make up 74 percent of the combined mass-energy in the entire universe. In comparison, ordinary matter accounts for only 4.6 percent.

"The discovery of dark energy was extremely surprising, and is I think the greatest discovery it helped make," said astrophysicist Mario Livio at the Space Telescope Science Institute, the science operations center for the Hubble Space Telescope. "And we still don't have any idea really what it is. The nature of this dark energy is at some level the biggest problem that physics is facing today."

And Hubble did not only make the universe seem a larger place by showing us it was growing — the orbiting telescope also hinting there was a lot more for us to learn.

"It's generated a tremendous sense of humility because we've discovered how we understand so little about the universe, from dark energy to dark matter to how galaxies can change across 13 billion years of cosmic history," said Space Telescope Science Institute director Matt Mountain. "It's completely changed our perspective on the universe."

Hubbles surprising finds

When Hubble was launched, one of its main missions was discovering when the universe was born. Before the orbiting telescope was deployed, it was highly uncertain as to how old the universe was, which could lead to laughable possibilities, such as stars older than the universe.

By measuring where distant galaxies are more accurately than ever before and how fast they are moving, Hubble greatly narrowed down the rate at which the universe is expanding, helping refine estimates of the universe's age down to roughly 13.75 billion years. However, by solving the mystery of the universe's age, it unexpectedly turned up an even more profound enigma — the universe's expansion is inexplicably accelerating, instead of slowing down as one might expect due to the pull of gravity from galaxies.

There were earlier suggestions that a "cosmological constant" might exist with the effect of a repulsive force that acted against matter's gravitational attraction, with the most notable proposal coming from Einstein. Before Hubble, however, "without observations, no one took those speculations particularly seriously," Livio said.

The promise of dark energy

Solving the mystery of dark energy could revolutionize physics. It 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. It has also prompted speculation regarding the universe's fate, raising the possibility that dark energy ends the universe in a Big Rip.

Still, much remains unknown about dark energy. One idea is that it literally comes from empty space — from energy that quantum mechanics theorizes should exist in vacuum.

The problem is that preliminary calculations as to how strong dark energy might be if it was a consequence of vacuum energy were an astounding 120 orders of magnitude greater than we actually see with dark energy. That is a 1 with 120 zeroes behind it.

"Even if you refine estimates further, you still miss the mark by more than 50 orders of magnitude, which is again ridiculous," Livio said. "Another possibility is that it's some sort of field, but we don't understand why that field should be there, and whether it might be related in some fashion to what caused inflation of the universe at its very beginning. A third possibility is that there's really no dark energy at all, but that we have to change our theory of gravity, that Einstein's theory of general relativity is not correct when we get to larger scales of the universe."

In each of those cases, "we're talking about a fundamental change in our understanding of physics, the very basic physical theory that governs the universe," Livio noted.

Not since Galileo

"It's fair to say that when you look back at history, Hubble will have had as much impact as Images - 20 Years of the Hubble Space Telescope