What is the universe?
The universe is everything. It includes all of space, and all the matter and energy that space contains.
It even includes time itself and, of course, it includes us.
Size?
The universe is believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; it has been expanding since its creation in the Big Bang about 13 billion years ago.
Now, that’s information in totality is over whelming. Let’s begin with a space pebble, floating around space until it’s caught by the Monster … gravity.
Stirred, not shaken: How pebbles become planets
(New theories for the curious)
First, let’s downsize from the Universe, to our solar system, to the planets know as Earth, and Mars.
It was once believed that Mars and Earth were of similar size. But not so. Mars has only 10 % of the mass of the Earth and that has been a long-standing puzzle for solar system theorists. In the standard model of planet formation, similarly sized objects accumulate and assimilate through a process called accretion; rocks incorporate other rocks, creating mountains; then mountains merge to form city-size objects, and so on.
The structure of the inner solar system is actually the natural outcome of a new mode of planetary growth known as Viscously Stirred Pebble Accretion (VSPA). With VSPA, dust readily grows to “pebbles” — objects a few inches in diameter — some of which gravitationally collapse to form asteroid sized objects. These primordial asteroids feed on the remaining pebbles, as aerodynamic drag pulls pebbles into orbit, where they spiral down and fuse with the growing planetary body. This allows certain asteroids to become planet-sized over relatively short time scales.
However, these new models find that not all of the primordial asteroids are equally well-positioned to accrete pebbles and grow.
“This means that very few pebbles collide with objects near the current location of Mars. That provides a natural explanation for why it is so small,” said Kretke. “Similarly, even fewer hit objects in the asteroid belt, keeping its net mass small as well. The only place that growth was efficient was near the current location of “Earth and Venus.”
“This model has huge implications for the history of the asteroid belt,” said Bottke . Previous models have thought that the belt originally contained a couple of Earth-masses’ worth of material, meaning that planets began to grow there. The new model predicts that the asteroid belt never contained much mass in bodies like the currently observed asteroids.
Poor Mars, left to be anorexic because of Gravity. And the rest of what I heard was “Blah, Blah, & Blah.”
So I dummied it down to my level.’
First, let’s downsize to, say, our solar system. Then let’s go further and think of the asteroid belt and the planets Earth and Mars.
Back in the day, it was believed that these 2 planets were about the same size. Well, they aren’t. Why? One theory is that similarly size planets grow by a process called Accretion. That means that rocks combine with other rocks on and on, until they eventually becoming an asteroid.
New calculations by Levison and co-authors Katherine Kretke, Kevin Walsh and Bill Bottke, all of SwRI’s Planetary Science Directorate follow the growth and evolution of a system of planets.
These learned Scientists believe that the structure of the inner solar system is actually the natural out come of a new mode of planetary growth known as Viscously Stirred Pebble Accretion (VSPA). (How imaginative)
With VSPA, dust readily grows to “pebbles” — objects a few inches in diameter — some of which gravitationally collapse to form asteroid-sized objects. These primordial asteroids feed on the remaining pebbles, as aerodynamic drag pulls pebbles into orbit, where they spiral down and fuse with the growing planetary body. This allows certain asteroids to become planet-sized over relatively short time scales.
“Well, after all that effort, sigh”. Several billion years from now, the Sun will expand, swallowing Mercury and Venus, and filling Earth’s sky. It might even expand large enough to swallow Earth itself. It’s difficult to be certain. After all, humans have only just begun begun to understand the cosmos.
Put another way, the matter, energy and everything in the universe(including space itself) was more compact last Saturday than it is today.*
But, that’s a subject for another day 
Garnet 