Hi all! I'm new to this forum and, as a matter of fact I only recently rediscovered my interest for physics. I am starting to catch up, yet I find that Wikipedia sometimes falls sort for some basic questions, and I was hopping that this community could gently help me understand some concepts and suggest books and sources to continue my learning. Please forgive my boldness posting a question given my lack of academic training and my thin knowledge of physics and cosmology. Yet I am very excited that this is my very first post. The question at hand is: I was reading about one of the not very well explained mysteries of the current state of science, which is why the expansion of the universe is accelerating when there was a phase in which it was decelerating. I have read that it has been postulated the existence of dark energy which would accelerate the expansion of the Universe. Lets assume that it exists (in the postulated way or an equivalent mechanism). For the sake of this discussion the nature of the dark matter (or equivalent mechanism) is not relevant. My question is very simple. Where did this dark energy (or equivalent mechanism) come from? We know that there was a phase in which the expansion was decelerating. Which means that the balance of all effects controlling the expansion (whatever dark energy was present then in the Universe and whatever other effects) was negative. Yet now we observe that this balance is positive, causing the acceleration. In a rough way, this would lead to following four possibilities: (1) The amount of dark energy has increased (where from?) (2) The strength of dark energy has increased (how, given that this would break temporal isomorphism and maybe other symmetries?) (3) The amount of sources of negative acceleration has decreased (where did they go to? and, wouldn't be this easily observable?) (4) The strength of sources of negative acceleration has decreased (how, given that this would break temporal isomorphism and maybe other symmetries? and, wouldn't this be easily observable?)) I was wondering if someone can tell me if this is already into consideration in the existing models, and how do they address this. Or maybe I am missing out something essential. In any case, if someone can bring some light, preferably referring to sources, I will be very grateful. Thanks in advance!