Looks like dark matter might throw a monkey wrench into this:
http://m.phys.org/news/2015-11-dark-fun ... tants.html
This article is comprehensive, up to date and nontechnical. A finer article I couldn't find and it explains why space can't be empty:
https://aeon.co/essays/why-is-the-speed ... d-of-light
PhilX