Does the law of conservation of energy mean that all photons and matter particles that exist in the universe right now have, in one form or another, always been around from the Big Bang on (only closer), and there will never be an increase in the amount of energy/mass? Doesn’t dark energy contradict the law of conservation of energy?

No, for multiple reasons.

First of all, photons and other bosons do not have number conservation. Photons can appear and disappear. "in one form or another" is a bit vague.

Second, conservation of energy is only valid in a particular reference frame, and the universe is not just one reference frame. That makes it difficult to apply the concept to the whole universe. 

Here are a coupe of articles on the difficulties with defining energy in GR (i.e. for the universe):

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ (this one is about the issue of dark energy)

However, we don't even know what dark energy is, or why its effects appear to have increased.

Thanks @swansont and @Strange, I will look into those articles.

Edited November 24, 20178 yr by MarkE