How hot is dark mat­ter?

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com­pet­ing the­o­ries sug­gest that dark mat­ter may be cold, warm or hot. the tem­per­a­ture refers to par­ti­cles’ speed. cold dark mat­ter par­ti­cles sit still, like wa­ter mol­e­cules frozen in ice; hot dark mat­ter par­ti­cles zip about like mol­e­cules in steam, spread­ing into their sur­round­ings.

the speed of dark mat­ter par­ti­cles mat­tered when they emerged from the Big Bang. set loose in com­puter sim­u­la­tions, cold dark mat­ter par­ti­cles don’t move. Grav­ity pulls them to­gether, cre­at­ing lumps in the pri­mor­dial soup. hot dark mat­ter par­ti­cles whiz around near the speed of light, stir­ring it up.

for­tu­nately for us, most real dark mat­ter was tepid. it cre­ated lumps that even­tu­ally grew into hab­it­able gal­ax­ies like the Milky Way. con­fus­ingly, the the­o­ries’ names have noth­ing to do with the present speed of dark mat­ter par­ti­cles. our uni­verse ex­panded so rapidly that the ini­tial mo­tion of any dark mat­ter par­ti­cles be­came ir­rel­e­vant. then par­ti­cles fell into gal­ax­ies, pick­ing up speed. Dark mat­ter par­ti­cles in the Milky Way now move 20 kilo­me­tres (12.4 miles) ev­ery sec­ond. that’s twice as fast as hy­dro­gen on the sur­face of the sun.

un­der­ground par­ti­cle de­tec­tors are look­ing for cold dark mat­ter par­ti­cles known as WiMps, but con­tro­ver­sial ev­i­dence has re­cently emerged for ster­ile neu­tri­nos, a type of warm dark mat­ter. a satel­lite to map this warm dark

mat­ter was lost in March, keep­ing the de­bate open for now.

Richard Massey is a physi­cist work­ing as a Royal So­ci­ety re­search fel­low in the In­sti­tute for Com­pu­ta­tional Cos­mol­ogy at Durham Univer­sity

the­o­ries es­ti­mate that about 26.8 percent of the uni­verse is dark mat­ter

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