20 ASTRONOMY • MAY 2020
Spiral galaxy M101 harbors
hundreds of Cepheid variables
that scientists use to measure
the universe’s expansion rate,
though the value differs from
that deduced from the cosmic
microwave background. M
also harbors vast amounts of
mysterious dark matter.
Enigmas such as these have
cosmologists questioning their
ideas on cosmic evolution. This
composite image combines
visible light (yellow), infrared
radiation (red), and X-rays
(blue). NASA/ESA/CXC/SSC/STSCI