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Carl Brennan stayed in his seat after the planetarium show , his son asleep against his coat . On the way in , the boy had asked what the dark was made of , and Carl had promised an answer after . He still did not have one he trusted . He wanted a single sentence for breakfast , something he could say without watching the boy's face go puzzled . The presenter had used dark matter and dark energy in one breath . Carl had already folded them together . Priya Raman was coiling a cable by the rail . She had been the voice in the dark . He went down with the boy heavy on his shoulder , careful on the steps . " Most of the universe is this dark stuff , " he said . " One thing . I can tell him that and stop . " The boy sighed and did not wake . Priya kept her voice under the hush of the dome . She wanted this man to walk out with two problems , not with a softer fog . " In the accounting we use now , ordinary matter is not most of the universe . The kind in stars , and in us , is the smaller part . That much you can tell him . Do not tell him the rest is one thing . Dark matter is the name for extra mass . If we use ordinary gravity , galaxies rotate as if more matter is pulling on the outer stars than the bright parts can account for . We infer the mass . We have not scooped it into a jar , and we do not have its name as a particle . Dark energy is a different placeholder . It is what we are pointing at when we say the expansion of space is speeding up . It is not more of that unseen mass . " Carl shifted the boy so the coat would not slip . " He will ask me which one is the real dark . " " Then give him two sentences and do not glue them , " Priya said . " There is extra mass we cannot see . The expansion is speeding up . If you make those one filling , you will have taught him a merger nobody has actually found . I am not going to trade you a smoother sentence . " A door at the back of the dome shut . Carl looked at his son and said the two sentences under his breath , with a pause between them .
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