IMO, the Void is essentially a dimension consisting of many different forces manifesting as fundamental principles(Lohk, Netra, Jahu, etc.). Some of these aspects seem to be much more dominant than others and in some cases diametrically opposed to each other, like Order and Chaos, Vome and Fass. The Deimos worms exemplify that well. While other aspects may be associated with this as well, certain colors are closely linked to these aspects. This is most notable with Order and Chaos, blue and red. This color coding also extends beyond, if you look at the color of health vs. energy orbs, or a Tenno's void beam attack changing colors from blue to red and back as when their Shadow overwhelms them in the Rell minicomic. The Veil in Railjack missions has two distinct skyboxes. One red and misty(chaotic), the other bright and focused like a star(ordered).
Cephylon Cy mentions that the container the finger on the railjack is placed in, the 'Orgone Accumulator' had been resealed, and that as a consequence Negentropy levels have fallen to safe levels. Orgone is an esoteric concept and said to be akin to life force, while negentropy is a term describing how far removed a given state is from normality. This further solidifies the perception that the Void as a whole is a space of pure potential where thoughts and emotions can take shape and are able to influence the process of creation itself, unbound by the rules of the material universe.
In short, the Void may be as close to what we imagine as the quantum realm as possible. An endless see of fluctuation, energy creating and destroying matter within a unfathomably short amounts of time. So, what's the opposite of this ever changing sea of pure creation and potential? What'd be the opposite to the Void?
A completely lack of it. No creation, no destruction, no movement, nothing. In one word: Indifference. That's what I think the Anti Void refers to.