Answer 1:

I'm not sure if other scientists would answer this question the same why I do, but I think this is a very philosophical question that depends on what you mean by matter. If you are asking if light carries energy and momentum, the answer is definitely yes. But perhaps you mean: is light a particle?

Quantum mechanics makes this question a little murky: everything, including light, in quantum mechanics is both a particle and a wave (really, it is a definite something that has both wave-like and particle-like properties). So if you think matter is something that is made of particles, either everything is or isn't matter depending on how you look at it. In fact, I think that is the wrong way to think about it. What you really want to know is: does light have properties that other kinds of particles don't? The answer is yes. Most of the particles that make up everything we traditionally call matter are called "fermions". Things like photons are called "bosons". There are many properties not shared by fermions and bosons - one is that two fermions cannot exist in exactly the same state. Two bosons, on the other hand, can exist in the same state. Though there are composite particles made up of smaller fermions that behave as bosons, the fundamental particles that are bosons - we think they're fundamental, anyway - do behave differently than the fundamental particles that are fermions. To make a long story short, these differences in properties are responsible for the way these particles manifest themselves at large scales in the everyday world, where quantum mechanics is not important. A boson appears as some kind of field or wave, and can be directly linked to one of the four forces (electromagnetism, gravity, the strong force, and the weak force) whereas fermions form individual particles such as atoms. So, there is definitely a sense in which light is very, very different than electrons or protons. I'm not sure if I would call light matter or not, however. Certainly it can do some of the things you would think only traditional matter can do - like carry momentum and transfer it in a collision. But it certainly has some properties that are fundamentally different than the stuff that makes up traditional matter (things that are made of atoms).