This Post is a departure from my normal posts as it is about Theoretical Physics.  One of my fascinations about science is the origin of the universe and the theorized existence of Dark Matter and Dark Energy.  These concepts have gained more attention and popularity among scientists and science buffs in recent years.  What I am about to describe is for those who know little to nothing about these concepts.  Some of the discussions are not entirely scientifically accurate because very complex concepts involving very high level math are being greatly simplified to make them easier to understand as well as my own limited understanding.

The Big Bang (BB) Theory is often referred to by scientists as the Big Bang Inflation Theory due to the instantaneous detonation of the most short-lived massive repulsive force ever in our universe which caused the universe to instantly expand or Inflate homogeneously and isotropically a nano-moment after the BB.  This set the stage of its future expansion and the formation of all the heavenly bodies and substances.

The BB was essentially a two-step processes after Inflation which resulted in the interplay between the forces of Dark Matter and Dark Energy as they and the universe were being formed.  The BB occurred about 13.8 billion years ago (calculated by backwards extrapolation) when an unimaginable amounts of energy instantly appeared from nothing at one point (refered by scientist as a ‘singularity’) in space at the instant of the Biggest Bang ever.  Singularities are theoretically possible according to Einstein’s Theory of General Relativity though what initiates them is unknown.  This energy was propelled outwards by Inflation.  The glare of the BB was so intense that the heat radiated would have rendered any matter into pure energy if it had existed even hundreds of billions of miles away.  As the universe expanded it started to cool down perhaps 300,000 years later forming all the atoms and elements that constitute matter today.  Tens of billions of Galaxies were created each with hundreds of millions or billions of stars and other matter and energy over billions of years.

But the gravitational attraction between galaxies decelerated this expansion until about 5 billion years later when it started to mysteriously expand again with ever-increasing speed from some mysterious force. Though the BB theory has been quite popular in the last couple of decades other theories are starting to pop up related to string theory which I still struggle to understand.

Dark Matter is the matter in galaxies which is about 23% of the entire universe, the rest of the visible universe that we see being only 4%.  A couple of astronomers 50 years apart noticed that clusters of galaxies rotating around each other and galaxies rotating around their own axises furthest from the center rotated at far higher rates than could be accounted for by the mass and gravitational pull of all the observable objects within.  To account for this unexplained rotational speed scientists hypothesized Dark Matter which is transparent to light but has mass and gravity that could account for this higher overall mass and rotational speeds.

Dark Matter has since been indirectly detected en-mass by the bending of light (gravitational lensing) observed around areas in galaxies where there are no observed masses large enough to bend light and has been mapped out in our own galaxy.  It is more dense in some areas than others being somewhat clumpy and unevenly distributed.  It extends out some distance from the observed physical galaxies and is primarily what holds galaxies together through its gravitational pull.  Scientists have yet to detect the presence of a Dark Matter particle.

Dark Energy is far more difficult to explain because it does not relate easily to everyday experiences and is not well understood.  It is very different from Dark Matter.  The great astronomer Hubble discovered that distant galaxies were moving increasingly faster away from each other the further apart they were like spots on an inflating balloon similar to the homogeneity and isotropy of Inflation.  More recently the Hubble Telescope has revealed by observing the most distant galaxies that long time ago the universe was actually expanding more slowly than it is today meaning that expansion is accelerating.  There appears to be no center from which it started expanding such as the location where the BB started.  It is as if the origin of the universe is at the center of each galaxies.  The more distant galaxies are apart the faster they seem to be moving apart regardless of which galaxy or direction you looked.

To explain this phenomenon scientist hypothesized Dark Energy which is expanding with the universe carrying galaxies along with its homogenous and isotropic expansion.  Unlike Inflation the force of Dark Energy is extremely weak but like Inflation is homogeneous and isotropic everywhere in the universe.  However the cumulative force of Dark Matter between galaxies can be considerable.  A very strange thing happens as the universe expanse, more Dark Energy is created everywhere to maintain constant density of Dark Energy thus nudging galaxies away from other galaxies.  Unintuitively according to General Relativity of time-space, energy and matter do not have to be conserved.  That is why the singularity of the BB was possible.

For the first 5 billion years Dark Matter’s gravitational pull slowed down the exploding Inflation effects of the BB.  But gravity gets weaker as matter is spread over a larger expanding volume of the universe and eventually the constant weaker force of Dark Energy everywhere starts to dominate dragging galaxies further apart from one another until now 8 billion years later Dark Energy is the overwhelming dominating force expanding the universe.

Dark Energy is about 73% of the universe, the difference between the entire universe 100% and the sum of Dark Matter 23% and the visible universe 4%.  If the universe continues to forever expand as many cosmologists believe galaxies will speed apart faster than the speed of light becoming invisible to each other leaving a sky only lit by the glow of stars within each galaxy until all the stars eventually die leaving a forever cold and dark expanding universe.

In truth science knows almost noting about Dark Energy or how the universe will end.  The label Dark Energy exist more as a Post-it to account for the mysterious repelling force between galaxies until scientist “boldly go where no man has gone before.“-StarTrek

Definitions:

Universe:
All existing matter and space considered in the cosmos since the Big Bang.

Galaxy:
A massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter.

Homogeneous and Isotropic:
Homogeneous – means the reference phenomenon is the same everywhere in the confines of the universe.
Isotropic – means the reference phenomenon is the same in all directions in the confines of the universe.

Inflation:
An event that occurred immediately after the Big Bang which lasted about 10⁻³³ seconds an extraordinary short period of time when a sudden surge of the largest repulsive force ever  ejecting energy homogeneously and isotropically throughout the, at that time, grapefruit size universe.