Scientists are pretty sure the universe contains more matter than the stuff we can see, and their best guess is that it takes the form of invisible particles. (europeanspallationsource.se)
According to the standard paradigm, 23 percent of the mass of the universe is shaped by invisible particles known as dark matter. (sciencedaily.com)
The Axion Dark Matter Experiment just entered the most sensitive phase yet in its search for invisible particles to explain the universe's hidden mass (scientificamerican.com)