Physics/Astro Physics Hub : [ Physics/Astro Physics Deep Dive (v1.1) ]   Welcome to the definitive guide to the cosmos. This series of essays explores the fundamental rules governing our universe, tracking the journey from the cosmic macro-scale down to the strange quantum world. Cosmic Beginnings & The Fabric of Space: These essays examine the birth of the universe, how it expands, and the invisible scaffolding that shapes everything we see. [ Big Bang and Cosmic Dawn (v1.1) ] -   How the universe transitioned from a hot, dense singularity into the structured cosmos we observe today. [ Space-Time Fabric (v1.0) ] -  Understanding Einstein’s revolutionary idea that gravity is not a pulling force, but the bending of geometry itself. The Expanding Universe :   Cosmic acceleration, dark energy, and the mysterious forces pulling the universe apart. [ Dark Energy (v1.1) ] - what is causing the universe to expand faster [ Dark Matter (v1.1) ] - what are the clumps that ...

Physics can get awfully close to the universe creation event, but it is very unlikely it can say anything about why the Big Bang occurred and what it was before it. It is not even clear causality works in such instances. This is as close as we can get to God's creation. At time zero, there was no space, time, or matter. It is assumed there was a gravitational singularity. Here is an account of the progression of the universe from “Physics of the Universe” and other sources. This is basically the standard model of cosmology. Since the Big Bang, 13.787 billion (± 20 million) years ago, the universe has passed through many different phases or epochs. Due to the extreme conditions and the violence of its exceedingly early stages, it arguably saw more activity and change during the first second than in all the billions of years since. The first second of creation is dominated by particle physics and quantum mechanics and may not make much sense to a layman. To better understand it, you ...

  Much of this essay is a summary of classes of Prof Richard Wolfson of Middlebury College among other sources. The Quantum and Relativistic Revolutions Einstein developed the special theory of relativity in 1905 and further expanded it to the general theory in 1915. Special theory is, in its essence, a great equalizer. It says that your frame of reference does not matter - it could be at rest or moving at close to the speed of light. Physics experiments give the same result. You play the same tennis on a steady cruise ship or on land. You eat peanuts the same way on a steady airplane or on land. This simple thing leads to some interesting effects and fundamentally alters classical physics. The general theory of relativity turns out to be a theory of gravity. After that quantum mechanics was developed. The key concept of quantum mechanics can be illustrated by emptying a jug of water repeatedly in half until you cannot do it anymore. You are left with one molecule of water....

Two of the biggest mysteries in astrophysics are dark matter and dark energy. Neither are understood. 27% of the universe is estimated to be dark matter (this works out to 80% of matter in the universe) and 68% of the universe is estimated to be dark energy. That is a lot of the universe that is not understood!! We understand only the 5% remaining matter!! There are strong arguments in favor of the fact that a significant part of the matter in the Universe does not emit anything and is therefore invisible. The presence of such invisible matter can be found by its gravitational interaction with radiating matter. The study of galaxy clusters and galactic rotation curves indicates the existence of this so-called dark matter. So, by definition, dark matter is matter that does not interact with electromagnetic radiation, that is, it does not emit it and does not absorb it. Its nature is unknown.  Dark energy is a theoretically postulated repulsive force that counteracts gravity and caus...

A key reference is Space.com. This is largely an extract.  Dark matter is the mysterious stuff that fills the universe, but no one has ever seen it. It is believed to be 80% of the matter in the universe. We only assume it exists because, without it, the behavior of stars, planets and galaxies simply wouldn't make sense. Their observed motions do not match their detectable mass. If you include both matter and energy dark matter is 27% of the universe's content with normally detectable matter being 5% of the universe's content and dark energy being 68%. Both dark energy and dark matter are not understood. I wrote an essay on dark energy that covers that subject:  Dark Energy What we do know about dark matter is that if we look at a typical galaxy, take account of all the matter that we see (stars, gas, dust) and use Newton's Laws of Gravity and motion (or, more correctly, Einstein's General Relativity - GR), to try to describe the motions of that...