What is the universe made of? Astronomers face an embarrassing conundrum: they don't know what 95% of the universe is made of!

the universe is composed almost completely of dark energy and dark matter and ordinary matter.

Other contents are electromagnetic radiation and antimatter

Astronomers face an embarrassing conundrum: they don’t know what 95% of the universe is made of. Atoms, which form everything we see around us, only account for a measly 5%. Over the past 80 years it has become clear that the substantial remainder is comprised of two shadowy entities – dark matter and dark energy. The former, first discovered in 1933, acts as an invisible glue, binding galaxies and galaxy clusters together. Unveiled in 1998, the latter is pushing the universe’s expansion to ever greater speeds. Astronomers are closing in on the true identities of these unseen interlopers.

Explanation:

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increased stability in the completion of fluorine's outer shell.