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Almost 30 percent of the total matter-energy budget is said to be composed of so-called cold dark matter and almost 70 percent of "dark energy," leaving only about 4 percent as visible matter in the form of the atoms that make up the stars, planets, interstellar dust, and ourselves. Such is the degree of discrepancy between theory and observations today.

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Could it be, nevertheless, that Einstein's theory is wrong? Might it be necessary to modify it—to find a new theory of gravity that can explain both the stronger gravity and the apparent antigravity being observed today—rather than simply throwing in invisible things to make the standard model work?

Today, like the elusive planet Vulcan in the nineteenth century, dark matter is accepted by the majority of astronomers and physicists as actually existing. Dark matter, although it has never been seen, is part of the generally accepted standard model of physics and cosmology, which also includes the big bang beginning of the universe.

It is difficult to falsify the hypothesis of dark matter, because, as with Ptolemy's epicycles, true believers can always add additional arbitrary features and free parameters to overcome any conceivable difficulties that occur with the dark matter models.