particle physics - Where does matter come from?

I admit, it's been a few years since I've studied physics, but the following question came to me when I was listening to a talk by Lawrence Krauss.

Is there any knowledge of from where matter that exists today originated? I recall that the law of conservation of mass asserts that matter cannot be created nor destroyed, but surely the matter we see today had to be created at some point? Perhaps I am applying this law in the wrong fashion.

The reason I ask, is because Krauss mentioned that the elements of organic matter where created in stars, not at the beginning of time (whenever that may have been), but I ask, where did the building blocks for these elements arise? Were they too created in stars? If so, from where did their constituent building blocks come?

Please forgive me if this off topic, it is my first post on this particular stackexchange site. Thank you.

Answer

The law of conservation of mass is only valid in the classical limit. More in general, the combination of mass and energy is conserved, as they can be exchanged under certain conditions $E=mc^2$.

In very simplified terms, it is currently understood that, initially, possibly only energy was present. Then this energy started to partially change into mass, forming first quarks and electrons (mostly). Then heavier particles. Finally, once the universe cooled down a bit more, the first atoms started to form through the aggregation of the particles. It is calculated that atoms would be split between hydrogen and helium with a 3:1 ratio - these are the two lightest atoms.

This was enough to create the first generation of stars, which with fusion and with their dying blast generated the heavier elements that build life.

This explains the currently understood principle of nucleogenesis - if you are instead more interested in how and why energy is changed into mass, and what is the nature of mass, then space_cadet's answer gives you a quick overlook of the hypotheses that are being studied.