Trentham and his colleagues released an image they say illustrates the effects dark matter can have on a galaxy, and which they say provides some observational support to their new idea. (See the click-to-enlarge near the top-right of this page.)

The image shows material streaming out galaxy UGC 10214, but the material heads toward nothing. Similar features, however, have been seen when one visible galaxy passes another. But there is no evidence that another normal galaxy caused this outflow.

"It's an unusual shape. Clearly something has happened there," Aizenman said. "But I think we need more evidence. You can't do it all on the basis of one image."

If dark matter is made of known particles, then improvements in existing observing techniques might root much of it out. Brown dwarf stars, for example, are thought to be nearly as prevalent in the universe as real stars. But few of these failed stars have been found, likely because they are small and dim.

In infrared wavelengths and with space-based observatories, more brown dwarfs could be detected. The same could be true for black holes, the researchers said. And if dark matter is normal matter, then a dark galaxy might act as "gravitational lens," distorting the appearance of distant galaxies.