What is the Universe made of...?

We often think that all of big questions in science have been answered.... food for thought below...

Astronomers scouring the heavens with powerful telescopes can see objects that are billions of trillions of miles away. These observations have proven essential to piecing together a fairly refined picture of the history and evolution of the cosmos. Nevertheless, a gaping hole remains in our understanding of a basic question: What is the universe made of? For more than 100 years we’ve known about atoms, and over the past century or so we’ve gone further and identified atomic constituents like electrons and quarks, as well as their exotic cousins - neutrinos, muons, and the like. But there is now convincing evidence that these ingredients are a cosmic afterthought. Current data shows that if you weighed everything in existence, these familiar particles would amount to about 5 percent of the total. Most of the universe is composed of other stuff, which, with all of science’s deep insights, we’ve yet to identify.

How do we know this? Well, over the course of many decades, astronomers studied the motion of galaxies and the stars within them, and found that the gravity exerted by this luminous matter was insufficient to account for the way these heavenly bodies moved. Only by positing large amounts of additional matter that doesn’t give off light (visible, x-ray, infrared, or any other kind) and is thus invisible to telescopes, could the data be explained. Through detailed cosmological measurements, scientists also discovered that this so-called dark matter couldn’t be made of the same electrons, protons, and neutrons that make up everything with which we are familiar.

Then, in the late 1990s, two groups of astronomers, one led by Saul Perlmutter of the Lawrence Berkeley National Laboratory, the other by Brian Schmidt of the Australian National University, found something even stranger. Through observation of distant supernovas, these astronomers measured how the expansion rate of the universe has changed over time. Because of gravity’s relentless pull, most everyone expected that the expansion would be slowing. But the data from both groups showed the opposite. The expansion of the universe is speeding up. Something must be pushing outward, and luckily Einstein's general theory of relativity provides a ready-made candidate: A uniform, diffuse energy spread throughout space can act as an antigravity force. Since this energy gives off no light, it’s called dark energy.

Collectively, the observations establish that about 23 percent of the universe is dark matter and about 72 percent is dark energy. Everything else is squeezed into the remaining few percent.

Several experiments are now under way to identify dark matter. Scientists are searching for what they suspect is an exotic species of particle. Some studies are looking for clues by analyzing particles bombarding Earth from space; others, like the Large Hadron Collider, will analyze collisions between extremely fast-moving protons that have the potential to create dark matter in the lab. We are guardedly optimistic that we’ll be able to identify dark matter soon.

By contrast, the question of dark energy is wide open. What is its origin? What determined its quantity? Does the amount stay constant or vary? These are critical questions. Calculations show that if the amount of dark energy had been slightly larger, the universe would have blown apart so quickly that life as we know it could not exist.

- Brian Greene, author of The Elegant Universe