On a rubber mat

In the classical physics Newton's laws of gravitation explained gravity as a fundamental attractive force between any objects having mass. According to a legend Isaac Newton realized that there must be a force when an apple dropped from a tree. Around 1687 he presented that the force behaves same way between the Earth and moon or Earth and any object. Later the mathematical presentation of the gravitational force was determined as it's strength was proportional to the masses and inversely to the their distance square. Additionally, to make equation to work in the standard units a multiplier called gravitational constant was needed. By1862 James Clerk Maxwell described that electric and magnetic fields are generated by charges and currents. Analogously, Gauss's law for gravity presented gravity as a field caused by mass according to Newton's law in the classical physics. 

In the early 20th century the modern physics changed our understanding of the reality.  The theory of relativity especially reformed the gravity. In 1915 Albert Einstein published the general theory of relativity, that described gravity as a result of as a geometric property of four dimensional spacetime consisting of the three spatial and one time dimensions. Any mass or actually any form of energy stretches the spacetime around. If we make a two dimensional illustration we can think that space is a rubber mat where the Earth is on and presses the mat into the pit. The moon orbits the Earth since it does not have speed enough to "climb" from the pit. The spacetime geometry rules everything including light. Massive galaxies bend path of the distant light that is called gravitational lensing. Black holes are ultimate dense spots of mass and energy stretching spacetime over the limit that the speed of light is not enough to escape once entered the event horizon. The known origin of black holes is that an old massive star explodes and its core collapses by gravity. Based on the theory relativity, the spacetime stretches infinitely and the matter compresses to infinitely small singularity. The theory of relativity also predicted gravitational waves that are traveling disturbances in the spacetime. The first direct observation of gravitational waves was not made until 2015, when the waves generated by the merger of two black holes was received by the LIGO gravitational wave detectors leading to the Nobel Prizing. One more prediction was the frame-dragging of the spacetime. For instance, rotation of a massive object distorts the spacetime that gets twisted around the vicinity of the object.

During the modern physics era also the theory of quantum mechanics was developed. It focused on the atomic and subatomic level and one of the key findings was that energy and other quantities of a bound system are restricted to discrete values (quants). Up today, the standard model of particle physics describes three of the four known fundamental forces the electromagnetic, weak, and strong interactions, but not gravity. Whereas electromagnetic fields are quantified by photons, gravitation does not likely have such as quants called gravitons. Instead, gravitation is the curvature of the spacetime itself.  So far there are many theories describing gravity with quantum physics. One most popular is the string theory in which gravitational field is not quantized. Instead, in the loop quantum theory the spacetime is quantized and composed of finite loops woven into an extremely fine network having minimum discrete units for space and time.

The one of the other mysteries is the dark energy that is accelerating the expansion of the universe at the large scale. If the spacetime was a rubber mat and having pins on it representing galaxies, it would be stretched with increasing force so that all pins are getting away from each other. The longer is the distance between the pins the faster they escape from each other. The simplest explanation for the dark energy is the cosmological constant, a fundamental energy density of empty vacuum. Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over the interstellar space'. He actually first introduced the concept for making calculation for a static universe and later abandoned it after Hubble's confirmation of the expanding universe in 1931. However, in Einstein's unpublished work he wrote that more space needs to come into existence in order to keep the energy density constant. As a result, this form of energy would cause the universe to expand faster and faster as time passes. I would like take the zero-energy universe hypothesis (developed by Stephen Hawkins et. al) in the picture. It proposes that total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity that is basically potential energy. Further, when a black hole emerges, gravitational energy could increase absolutely bigger that the energy of the matter that created the black hole. My own hypothesis is that this excess of the negative energy would be compensated by a small rise of potential energy spread over the entire universe causing a energy density of the cosmological constant. Being like this in a large scale galaxies are sliding on the gravitational slope away from the empty space areas. This is naturally far from what we can make predictions on.

Hopefully this post sheds some light about the appearance and behavior of the universe. We are living in a complex and highly dynamic spacetime medium. To put it naïve simple it reminds a rubber mat that is curved and twisted around rotating masses, sometimes bizarrely infinitely, propagates waves like a membrane caused by astronomic collisions and boaters, and has very fine-grained and magically flexible structure being stretched in all dimensions more and more.