"Why would NASA want or need to lie about the visual properties of light in space? " Because they depicted the earth and moon as being a certain way when they supposedly went to the moon.
"Why would they pass up an opportunity to reap the glory of discovering something so earth shattering?" It wouldn't be a discovery, it would be an admission.
"...what benefit would they be harvesting from keeping this "fact" of light secret from the world? " They would have to explain what the sun REALLY is and that would mean revealing their deceit about the shape of the Earth.
"Yes, space is dark... but the sources of light in space are NOT. One need only look at them to see them (with the exception of gamma ray bursters, neutron stars, black holes, pulsars, etc..., which we already know to not emit visible light but, instead, UV, X-ray, Gamma and so on. Light through a dark void leaves said void dark in the absence of a target (wall, dust particles, a love letter to Mother Flat Earth) to reflect light back to our eyes. But it's not that the light is "invisible" in the more general sense. It only means that light is not currently acting upon our retinas."
No. Visible light literally is INVISIBLE in space. That is what the word means. You CANNOT see it. The light is there but it is invisible. Human retinas are created to see light at a certain speed. Just like our ears are created to hear sounds at a certain speed/frequency. The speed is too fast to detect in space, just like a dog whistle is too fast/high for us to hear.