Astronomers have discovered that they do not know what makes up over 90 percent of the universe. What is more, the discoveries that led to question their understanding of the fundamentals of physics itself. Of course, such questions are nothing new.
Toward the end of the 19th century, physicists observed something odd about the speed of light. They found that relative to an observer, light always traveled at the same speed no matter how fast the observer was moving. But that seemed to defy natural law! The problem was addressed in 1905 in Albert Einstein’s special theory of relativity, which showed that distance [length], time, and mass are not absolutes.
Then, after a flash of intuition that he termed “the happiest thought of my life,” Einstein began to develop his general theory of relativity, which he published in 1916. In this revolutionary work, Einstein wove gravity, space, and time together and refined the physics of Isaac Newton.
 THE GROWING UNIVERSE
Based on the evidence of the day, Einstein believed that the universe is static –neither expanding nor contracting. However, American astronomer Edwin Hubble, in 1929, presented evidence indicating that the universe is expanding or increasing.
Hubble also cleared up a long-standing mystery about certain fuzzy, luminous patches in the night sky, which were named nebulae because they appeared to be clouds of gas. But were all these nebulae within our galaxy, or were they outside it, as British astronomer Sir William Herschel [1738-1822] suggested over a century earlier?
When Hubble first estimated the distance to one of these entities, the Great Nebula in the constellation Andromeda, he concluded that the nebula was actually a galaxy a million light years away. That put it well beyond the Milky Way, which has a diameter of a “mere” 100,000 light years. As Hubble charted the distances to other nebulae, he began to unveil the enormous scale of the cosmos and triggered a revolution in astronomy and cosmology.
It was soon thereafter that Hubble observed that the universe is expanding, for he saw that distant galaxies were receding from us. He also noticed that the farther away the galaxy, the faster the recession.
Those observations imply that the universe of yesterday was smaller than that of today. When Hubble published his groundbreaking work in 1929, he paved the way for the development of the big bang theory of the origin of the universe, which indicates that the universe originated in a cosmic explosion approximately 13 billion years ago. But the picture is incomplete.
Since the time of Hubble, astronomers have been trying to measure as accurately as possible the rate of expansion, referred to as the “Hubble constant” if astronomers could calculate how fast the universe is expanding, they could use that calculation to estimate its age.
Moreover, the rate of expansion might have serious implications for the future. How so? It is reasoned that if, for instance, the universe is expanding too slowly, gravity might ultimately win out and cause everything to collapse in a final “big crunch”! But if the expansion is too rapid, the universe might expand forever and dissipate entirely.
While more precise measurements have provided answers to some questions, other questions have been raised –questions that cast doubt on our present understanding of matter and the fundamental forces of nature.
 DARK ENERGY AND DARK MATTER
Dark matter was postulated in the 1930’s and confirmed in the 1980’s. Today astronomers measure how much dark matter a cluster of galaxies may have by observing how the cluster bends light from more distant objects..
Researchers analyzing light from a special kind of supernova, or exploding star, found evidence that the expansion of the earth is actually accelerating! At first, the scientists were skeptical, but evidence soon mounted. Naturally, they wanted to know what form of energy was causing the accelerating expansion. For one thing, it seemed to be working in opposition to gravity; and for another, it was not predicted by present theories. Appropriately, this mysterious form of energy has been named DARK ENERGY, and it may make up nearly 75 percent of the universe.
Dark energy, however, is not the only “dark” oddly discovered in recent times. Another was confirmed in the 1980’s when astronomers examined various galaxies. These galaxies, as well as our own, appeared to be spinning too fast to hold together. Evidently, then, some form of matter must be giving them the necessary gravitational cohesion.
But what kind of matter? Because scientists have no idea, they have called the stuff dark matter, since it does not absorb, emit, or reflect detectable amount of radiation. How much dark matter is out there? Calculations indicate that it could make up 22 percent or more of the mass of the universe.
According to current estimates, normal matter account for about 4 percent of the mass of the universe. The two big unknowns –dark matter and dark energy –appear to make up the balance. Thus, about 95 percent of the universe remains a complete mystery!