For most of human history, time was thought of as something "absolute", in the sense that there was thought to be an absolute present, or "now", which was the same for the whole universe. This is highly intuitive, as in ordinary human experience, there is nothing to suggest that this is not the case.
However, advancements in physics and observational science in the 20th century actually demonstrated that this intuitive understanding of time is inaccurate. As objects in space approach the speed of light, time passes more slowly for them relative to slower-moving observers. And, photons, which move at the speed of light, effectively experience no passage of time.
This means that for an object that is traveling near to the speed of light, very little time would appear to pass at all. Even if an observer moving at a much slower speed was to observe the object moving for 10 hours, from the object's perspective, far less time (perhaps seconds) will have passed. Likewise, if someone were to travel at an extremely fast speed relative to earth, and then return after 80 years have passed on earth, they will not have aged 80 years, and 80 years will not have taken place from their perspective.
Therefore, because time can pass differently for different observers, there can be no absolute "now". If someone were to claim that something is happening "now", they would only be speaking from one perspective - theirs. They may in fact see distant events in completely different orders from other observers moving at different speeds relative to them, and to the events in question.
Given that there is no universal "now", what can be stated about what "currently" exists in reality? Some models of time state that all points in time are equally real, and equally "present" - including all "future" events - events which have not yet taken place from any perspective.
However, there is reason to doubt that this is true. Instead, an argument can be made that future events actually do not exist yet, and instead, are gradually brought into existence.
An argument for this view of time may go like this:
Notice that this argument concerns the future, but also has a lot to say about causality. Future events really do depend on a series of causes for their existence, and do not exist until those causes take place. On May 10th, Alice's body on May 11th does not exist. The cells in her body will change billions of times on May 10th prior to whatever configuration they find themselves in on May 11th. And, until those changes have actually happened from at least one perspective, the events that they go on to cause do not exist.
So, while there is no absolute present, there is also currently no future. Instead, reality is like an unfolding series of vectors in spacetime. Each atom can be thought of as one of these vectors, representing some "perspective". As events take place in from these "perspectives", they emanate out into reality as they come into view of other vectors. In this sense, time can be said to gradually "grow", or "unfold", from a multitude of perspectives. It is neither unfolding from one perspective in the sense of there being an absolute present, nor has it "already" unfolded in the sense of a static, completed block of all events.
This view also has implications for the past. The "past" is defined according to the perspective of the vectors that the event actually happened to. Other observers may observe the event much later, but that is irrelevant. Later observation from other perspectives does not mean the event happens later. Once the event has taken place from the perspective of the vectors it happens to, it is in the "past". This would mean that like the future, the past is not equally real at all times, as some theories suggest.
Any theory of time needs to take discoveries concerning relativity into account. This eliminates the idea of an absolute "now". However, this does not necessarily mean that all events currently exist. Whether an event does or does not yet exist is dependent upon causality, and the events in an event's causal chain must take place from some perspective before the event can be said to "exist".