Is it possible to slow down light? Last I knew, light is the fastest thing in the universe but I could be wrong.
Anyways, if the speed of light is 299,792,458 m/s (correct me if that is inaccurate) then is it possible to slow it down?
Scenarios
Will an invisible but super dense gas "slow" it down?
Is the speed of light microseconds slower on Earth than in the vacuum of space?
If we could reach absolute zero then would it affect the light's speed?
I am aware that black holes bend and "suck in" light but does the speed stay constant even though it's been rerouted?
 A: The speed of light in vacuum is 299,792,458 m/s - that is an unalterable quantity. However, light doesn't always travel in vacuum. The concept of a refractive index describes the relationship between speed of light in vacuum vs a particular medium, with the value for glass around 1.3 - meaning that the speed of light in glass is about 1.3x slower than in vacuum.
This is why lenses and prisms can work; it is also why the sun seems to change shape just before it sets over the sea; it is what causes the shimmering of the road ahead on a hot day - and many other optical phenomena.
In the atmosphere of Earth, light is indeed slower than in vacuum - the refractive index of standard air is approximately 1.00028, meaning light is about 0.03% slower in the atmosphere than in vacuum (but the exact value depends on wavelength, density, composition...)
A: No, the light was not slowed down. They made it interact and travel more. The slowdown in a refractive index material is as Floris and CuriousOne described. It is similar in for instance a light fiber, in essence a waveguide, where it slows down because of reflections against the Fiber edges or walls (i.e., material, so longer distance and also whatever time the interaction lasts). It's been well known for both refraction and waveguides. The rest about light slowing down in vacuum or different speeds for blue and red are nonsense. 
As for the Science article if it was not Science I'd ignore it, and the fact they misused the title is disappointing. I looked up another summary, the mask makes it take a longer path. See http://physicsworld.com/cws/article/news/2015/jan/22/structured-photons-slow-down-in-a-vacuum. 
The only thing interesting was they were able to slow down through the longer path single photons. This breaks no laws, and they used a dishonest title. Robert in a comment is also right. Photons can not travel at any speed except c. Everything else you doubt it. 
A: Yes the speed of light is not constant - it can be slowed down IN VACUUM. The discovery was published in 2015 in Science (but is unpopular, for obvious reasons):
http://www.upi.com/Science_News/2015/01/23/Scientists-slow-down-light-particles/1191422035480 
 "The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently."
http://science.sciencemag.org/content/347/6224/857 
 "Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860 
A: actually light moves at different speeds not only dependenig on the density of the environment but also the kind of light i.e red light is faster than blue, thats why stars seem to twinkle or change colour its because the defferent coulors rech your eyes at diferent times because of moving at different speeds
