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If at the dawn of the new 21st century, the global physics community had decided to hold a review lecture similar to Lord Kelvin's report a century ago, today's summary picture would be far less optimistic.

The two small cloudlets on the scientific horizon that worried scientists in 1900 had, by the end of the 20th century, grown not just into gigantic dark clouds of scientific ignorance but had also, one might say, obscured almost the entire universe from humankind.

More precisely, about 96% of the world around us constitutes something about which modern science can say practically nothing substantive. [o2]

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The only thing that has been achieved so far is to give the unknown components their own, not the best names: "dark matter" and "dark energy" (a more appropriate term might be the word "invisible").

Since dark matter, which makes up about 23% of all material in the universe, refers to particles, this ignorance falls under the category of quantum physics. In other words, what the first "cloud" eventually turned into.

Similarly, dark energy, which accounts for about 73% of the universe, turns out to be a direct offspring of another "cloud," known as Einstein's General Theory of Relativity (GTR). [o3]

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### Outside links

[o2] S. Matarrese, M. Colpi, V. Gorini, U. Moschella (Eds). *«Dark Matter and Dark Energy. A Challenge for Modern Cosmology»*. Springer (2011)

[o3] L. Papantonopoulos (Ed.) *«The Invisible Universe: Dark Matter and Dark Energy»*. Lecture Notes in Physics 720. Springer (2007)
