“Darkness” is the absence of photons- electromagnetic waves.
Electromagnetic waves can be expressed by its frequency, its wavelength, or Photon Energy.
Photons exist across a very wide range of energies and frequencies most of which we cannot see.
Microwave energy permeates the universe and microwaves are a frequency of photons (300 MHz- 300 GHz).
There are no places in the universe that are truly void of photons which we see as the cosmic microwave background and are truly dark.
There is “dark energy” and “dark matter” which do to its properties we cannot yet detect. However, microwaves permeate those regions of space as well. They simply pass through them and don’t interact with them.
Visible light as we see it ranges from 300nm ( blue) to 700nm,(red) or 430-770 THz.
A number of years back, I had a lens implant in my left eye. I noticed that when attending an event that used blacklight to illuminate objects painted to glow under that light, I could close my left eye and see only darkness- except for the glowing objects. But when I closed only my right eye, I could detect some of the previously invisible wavelengths, and spot the source of the light.
It was an interesting revelation.
Let’s do an experiment. For the speed of darkness as you perceive it let us go in to space. Space has no atmospheric scatter. Let’s say we go to the moon where there is no significant atmosphere. Darkness is perceived as instantaneous with the disappearance of the light source being obscured from view. Like turning off a light.
If you could back away into space and watch shadow envelop an area of the moon, you could watch it move across an area as shadow was created by the obstruction of the sun by the movement of the moon- sunset on the moon.
We watch sunset on earth but due to our atmosphere light remains scattered for some time after.
It’s a quicker experience during a total eclipse of the sun.
You can watch a total eclipse of the sun here in North America next year on August 21st, 2017.
The speed of the shadow, taking into account the speed of the moon around the earth (2288 mph) and the rotational speed of the earth will still exceed 1000 MPH!
Look for a place to view and book your accommodations early. Dry areas with low moisture are best, as clouds often form as the air quickly cools. Find a nice desert patch in the line if the eclipse. Avoid the coast with lots of moisture.