Since no-one knows for sure what Dark Matter actually is, or even if it actually physically exists beyond being an enticing theoretical construct, it's a bit difficult to compare it to anything else :)

Regarding the quote from Mr Wattles, he might be describing a concept commonly referred to (within metaphysical-style circles) as Thought-Form.

• The basic idea is that you start with Thought.

• If you focus thought enough, it becomes a kind of pseudo-matter called Thought-Form. It doesn't have the solidity of something physical yet it doesn't have the fleeting ethereal nature of pure thought.

• If you continue to focus Thought-Form enough, it becomes something perceptible to the physical senses of people other than the original thinker of the thought...a physical manifestation.

"Substance" is one of the basic terms in New Thought teachings from Wattles' time. "The formless, back of all forms." In the Seth material, this formless substance would be said to originate in Framework 2. "The origin of your universe is nonphysical, and each event, however grand or minute, has its birth in the Framework 2 environment. Your physical universe arose from that inner framework, then, and continues to do so." --Seth, The Individual and the Nature of Mass Events, Session 825

Framework 1 is the experienced physical world of "solid" manifestation.