Like all ga­lax­ies, most as­tro­no­mers be­lieve ours is filled with a strange, in­vis­i­ble sub­stance that be­trays its pres­ence only through its gravita­t­ional pull. The gal­ax­y’s stars would fly apart with­out this so-called dark mat­ter hold­ing them to­geth­er.

But the na­ture of “dark mat­ter” is a riddle—and a new study has only deep­ened the mys­tery. Now, “we know less about dark mat­ter than we did be­fore,” la­ment­ed Matt Walk­er of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., lead au­thor of a re­port on the work to ap­pear in the The As­t­ro­phys­i­cal Jour­nal.

Mod­els de­vel­oped by cos­mol­o­gists pre­dict dark mat­ter con­sists of some sort of ex­ot­ic par­t­i­cle that through gra­vity clumps to­geth­er with oth­ers of its kind. These ti­ny grains are be­lieved to be rath­er slug­gish, so dark mat­ter is typ­ic­ally dubbed “cold dark mat­ter,” slow par­t­i­cles be­ing nor­mally as­so­ci­at­ed with cold­ness.

Over cosmic his­to­ry, cos­mol­o­gists think clumps of dark mat­ter grew and at­tracted nor­mal mat­ter, form­ing the ga­lax­ies. Cos­mol­o­gists use pow­er­ful com­put­ers to sim­u­late this pro­cess. Their sim­ula­t­ions show that dark mat­ter should be densely packed in the cen­ters of ga­lax­ies. “If a dwarf gal­axy were a peach, the stand­ard cos­mo­lo­g­i­cal mod­el says we should find a dark mat­ter ‘pit’ at the cen­ter,” ex­plained Jor­ge Peñar­ru­bia of the Uni­vers­ity of Cam­bridge in the U.K., co-au­thor of the new stu­dy.

But mea­sure­ments of two dwarf ga­lax­ies show they con­tain a smooth dis­tri­bu­tion of dark mat­ter, Walk­er said. So “our mea­sure­ments con­tra­dict a bas­ic pre­diction about the struc­ture of cold dark mat­ter in dwarf ga­lax­ies.” In oth­er words, “the first two dwarf ga­lax­ies we stud­ied are like pit­less peach­es,” said Peñar­ru­bia.

Some as­tro­no­mers be­lieve dark mat­ter does­n’t even ex­ist, al­though the ma­jor­ity claim that many ob­served mo­tions of stars and ga­lax­ies are un­ex­plain­a­ble with­out it.

Dwarf ga­lax­ies are thought to con­sist of up to 99 per­cent dark mat­ter and only one per­cent nor­mal mat­ter, like stars. This makes dwarf ga­lax­ies ide­al sub­jects for dark mat­ter re­search­ers. Walk­er and Peñar­ru­bia an­a­lyzed the “dark mat­ter dis­tri­bu­tion” in dwarf ga­lax­ies neigh­bor­ing the Milky Way called For­nax and Sculp­tor. These each hold a mil­lion to 10 mil­lion stars, a mere hand­ful com­pared to the 400 bil­lion or so in the Milky Way. The team meas­ured the loca­t­ions, speeds and bas­ic chem­i­cal com­po­si­tions of 1500 to 2500 stars.

“S­tars in a dwarf gal­axy swarm like bees in a bee­hive in­stead of mov­ing in nice, cir­cu­lar or­bits like a spir­al gal­axy,” ex­plained Peñar­ru­bia. “That makes it much more chal­leng­ing to de­ter­mine the dis­tri­bu­tion of dark mat­ter.” The da­ta in­di­cat­ed that in both cases, the dark mat­ter is spread evenly over a huge re­gion sev­er­al hun­dred light-years across. A light-year is the dis­tance light trav­els in a year.

Some sci­en­tists have sug­gested in­ter­ac­tions be­tween nor­mal and dark mat­ter could spread out the dark stuff, but sim­ula­t­ions don’t show this hap­pens in dwarf ga­lax­ies, Walk­er and Peñar­ru­bia said. The new mea­sure­ments imply that ei­ther dark mat­ter is­n’t “cold,” or that it’s sur­pris­ingly strongly af­fect­ed by nor­mal mat­ter, added the re­search­ers, who are un­de­terred from their bas­ic as­sump­tion that dark mat­ter ex­ists. They hope to learn which of the two ex­plana­t­ions is bet­ter by stu­dying more dwarf ga­lax­ies, par­tic­u­larly those with an even high­er per­centage of dark mat­ter.