In its simplest form spacetime is the container in which all matter and energy in our universe exists. In this model gravitation is considered to be a consequence of mass affecting the geometry of the universe as opposed to a force that pulls things through it. To visualize this concept, it is common to use the cosmic grid analogy. Imagine a volume of space divided into a grid of cubes. In an empty universe, all the lines on this cosmic grid would appear parallel and at right angles to one another. However in the real universe, matter is concentrated in places like planets and stars causing a distortion in the spatial geometry around them.
Using the cosmic grid analogy, gravitational distortion causes all lines on the cosmic grid to bend inward toward planets and stars giving the impression that any other matter on the grid near them is being pulled towards them. When time is factored into the model, the geometry of the grid becomes dynamic, with all concentrations of matter and their surrounding geometry moving in relation to each other. Although the spacetime analogy has proven to be very accurate for most astronomical observations, it doesn't explain everything, mainly because it is only one component of a larger framework called the Standard Model, which in turn is based on something called the ( Big Bang Theory )
According to the Big Bang Theory, our universe expanded rapidly in all directions from a point source, and over the course of billions years it has grown very large while the energy inside coalesced into the stars, planets and other matter we see. Recently, observations have revealed that the most distant galaxies in our universe are accelerating away from those closer in. This was completely unexpected. After the Big Bang, there should have been no further force to propel them. Only gravity and inertia were thought to have played meaningful roles in defining the large scale motion of matter within the cosmos. However the observations of accelerating galaxies has caused scientists to consider previously unknown forces and have dubbed them dark matter and dark energy. There is no evidence that either actually exists, or if they do, what they are actually made of. They were merely factored into the mathematics to balance out the discrepancy between the math and actual observation.Such adaptations make cosmologists nervous because similar jury-rigging in the past has been a prelude to abandoning entire models. For example, many clung desperately to the geocentric model, going so far as to arbitrarily place all other planets in orbit around the Sun except Earth, but eventually the geocentric model collapsed under the weight of observations that could only be explained by the Copernican system. If the Standard Model succumbs to a similar fate, cosmologists, will be at a loss to explain the way our universe came into being and evolved to its present state.
However, even if the Standard Model should need revision, the concept of spacetime will likely remain a part of whatever the new model is. It is simply too convenient a way to describe the way things apparently are. Plus it makes predictions about the way certain things might happen. For example, spacetime geometry allows for a spatial shortcut called a wormhole, through which distant regions of spacetime can be connected. Science fiction writers and some ufologists have hypothesized that such a phenomenon might serve as a conduit, enabling widely separated civilizations to interact in a manner that would otherwise be difficult or impossible.