: Let me assure Mr. Mahony (dan@sidis.net) that I'm clear about what transparency is, and understand the concept of a dark galaxy  it consists of dark matter, which does not emit or absorb light or interact with "normal" matter or light in any way except gravitationally  as I stated, it is "invisible and essentially transparent". Sidis said the Milky Way (a "white space" region) was surrounded by "black spaces" that absorbed light. Dark matter doesn't absorb light. Therefore, dark galaxies are not the "black spaces" that Sidis postulated. Furthermore, the Milky Way is not surrounded by light-absorbing "black spaces"  we can see far beyond the Milky Way. In 1922, Ernst Öpik estimated the distance to the Andromeda galaxy to be about 450000 parsecs. On November 23, 1924, the New York Times published Edwin Hubble's findings: "spiral nebulae ... are in reality distant stellar systems. ... The number of spiral nebulae is very great". Sidis, in his 1925 work The Animate and the Inanimate, seems to have been unaware of these developments   he estimates the length of the "white brick/space" containing the Milky Way to be only 7000 light years. In other words, his theory was not even consistent with observations made before its publication. As far as I can tell, no peer-reviewed physics article has ever supported it. It's utterly inconsistent with what we now know. However, I didn't remove the section because the theory is bad  I removed it because it doesn't belong in this topic. Also, in my opinion, the theory had such little impact that there's no compelling need for a Wikipedia article about it  if someone feels differently, they're welcome to compose such an article and submit it to the usual editorial review process.