We know from the First Law of Thermodynamics that energy can not be created or destroyed. It follows from this law, that everything in the universe has always existed in some form.
Yet, I often hear scientific claims related to the "dawn of the universe" or "the beginning of time." Evidently, the universe is believed to be 13.7 billion years old, and, some scientists even hold that discussions of what happened before the big bang are "meaningless" - a non sequitur at best. Related to these claims is the common question of the form "when did the universe begin?" or "what started the universe?", etc. I have a few thoughts on this.
First, "time" is simply a standardized measurement or record of motion. It is a conceptual tool that allows us to define or identify relationships between other forms of motion or between causes and effects. In other words, time is a measurement made by humans, not something existing in the universe per se such as an atom or a rock. If nothing moved (or changed), there would be no time. If there were no humans, time would not "exist" out there.
If I walk from point A to B and it takes an hour, this is just the same as saying "during your motion from A to B, the earth spun 1/24 of a revolution." Expressing certain sequences of events in terms of a standard unit of motion is very useful conceptually. But, it is causality which gives us the sense of a direction of time, not time itself.
Perhaps, the fact that we are born, age, and then die creates a kind of bias towards the view that the universe must have started at some point or had some ultimate cause, but why does anyone insist on this? The universe doesn't know or care about time or our memories. There can be no "first cause" since everything has always existed. Entities, and therefore causes, can not spring from nothing.
Perhaps a better question to ask is: if the universe did "begin" then what was going on before it began, and how is that any easier to grasp and accept than simply following the evidence to the conclusion that everything has always existed?