Apparently, the question dates to the 1780s:, 50 years before Maxwell was born: https://en.wikipedia.org/wiki/Oh_Dear!_What_Can_the_Matter_Be%3F At 05:10 PM 7/18/2015, Tom Knight wrote:
http://www.theonion.com/article/astronomers-just-going-go-ahead-and-say-dark... <http://www.theonion.com/article/astronomers-just-going-go-ahead-and-say-dark-matte-50857>
On Jul 17, 2015, at 8:14 PM, Eugene Salamin via math-fun <math-fun@mailman.xmission.com> wrote:
The Bullet Cluster is a pair of galactic clusters that have collided. The gas clouds have interacted, and turned into an X-ray source from shock wave heating. The majority of the mass, as imaged by gravitational lensing, has passed on through without disturbance. This object is considered to be evidence for the actual existence of dark matter. -- Gene