It has been 2.5 million years since the first mass human extinction period, where humanity's rein of the Milky Way was pushed back to their home planet Earth. During those times, the supernatural supposedly existed, and was powerful enough for humanity to spread to the Milky Way without the need for technology and science.
Countless humans battled against powerful supernatural alien races and strange beings of supernatural power existed. An extreme, prolonged period of medieval times somehow existed, dating for billions of years. Evidence suggests such a thing, based on man-made structures in other star systems, and the fact that other humans were discovered on other planets.
All of this was guessed by very ancient temples, relics, and historic records of the past scattered throughout the Milky Way. None of it is confirmed, and is still speculation to this day.
In the end, most of the history of humanity before that is a mystery.
Most of humanity forgot about these things during the space exploration era early on when humanity finally put aside their differences and united. Wars happened, and many suffered, including the mixed alien races.
Eventually, a unified galactic government called the United Federation or UF for short, took hold of the Milky Way, and the lives of many improved. Science and technology continued to improve at staggering rates, and a prosperous era ensued. This was the peak of humanity at the time.
However, the massive population and demands eventually drained the galaxy of it's resources. The amount of planets drastically reduced as they were taken apart for construction of things like ring world, habitats, and space cities.
With the problem even affecting the Milky Way's economy, humanity had no choice but to venture to other galaxies. But that ended in failure.
To humanity's despair, an invisible barrier surrounded the Milky Way, called the Great Barrier. A solid barrier that formed a perfect sphere around the Milky Way, while still displaying the galaxies outside like the Andromeda.
Gradually, humanity entered another dark era where resource depletion became a major problem. The economy suffered especially for industry and mining. Humanity had no choice but to enter a low-power state in order to figure out a way to escape. Science and technology was given massive funding to solve this issue.
It wasn't until 200 thousand years later, or 1.2 million ADE, that humanity decoded the Great Barrier, and managed to create stable rifts to leave them. At the time, the United Federation was suffering from internal fighting and rioting from humanity who was suffering from being in a low-power state for so long.
To their despair, the galaxies that one could see through telescopes and the naked eye, were only projections by the Great Barrier. Past that, there was nothing but a pitch black void vacuum that stretched endlessly. Any probes that entered would vanish and be destroyed. At this point, humanity nearly fell apart and fell into a galactic war.
It wasn't until later that another portal had been opened up on a different location on the barrier. This one didn't show the same result like the hundreds of other ones, but instead another foreign galaxy!
The portal was extremely unstable, and they had to move the location until it was stable. The location being a little below the galactic orbit.
It was then that humanity invaded this new galaxy at a horrifying speed. This one was larger, and thankfully empty of intelligent life. It looked to be around the same age as the Milky Way, but with another Great Barrier as well. The galaxy was only named Galaxy 2.
It was then that the economy began flourishing once more from it's stagnant state. The mining and manufacturing industries that had been shut down for so long were opened to full power! An influx of resources once more brought humanity up from the dark ages.
When it was 1.3 million ADE, or a 100 thousand years later, the economy was back to it's peak, and even soaring past that. People who had been reliant on money and robots were put back to work again!
The galaxy was moving once more, and it stayed this way until 1.5 million ADE, when humanity finally managed to consume the galaxy of it's resources. At this point, the population was already around 10 sextillion and the United Federation was stable, free of rebellions and rioting.
The UF predicted another dark age and tried to create more portals, only to find the same result as before. Eventually, they found another stable portal entrance, on the opposite side of Galaxy 2. If one traveled from the Milky Way to Galaxy 2 and continued to the other side in a straight line, they could reach the portal.
Another galaxy, although very old, was on the other side as well. For another shock, back in the Milky Way, it was discovered that there was a galaxy on the opposite side of the other portal to Galaxy 2 as well.
This would continue till the current year. Humanity would discover relics and ancient temples from ancient races that had gone extinct in their own galaxy. At the same time, they would mine these galaxies dry of resources. Each galaxy was of different size and shape, while not too young or old.
Another discovery came out with the portal location patterns for each galaxy. They were not exactly opposite of one another, and instead actually curved to a slight angle. Scientists did a prediction, and figured out that the curvature would make a complete circle back to the Milky Way.
In a simpler term, imagine a wristband made of beads. It is made of many solid spheres by each other that make a circle. Imagine each sphere as a galaxy, and the predicted path being a circle. The results showed that it would form 108 galaxies, with the Milky Way being the 1st.
The galactic naming system was adjusted in the event that this stayed true. Humanity's home galaxy, the Milky Way was number 1. Galaxy 2, named Andromeda, had the number 2 added.
With the universal map showing the galaxies forming a circle, a galactic compass was formed. This compass always points to the center of the circle for each galaxy, allowing for west and east. The portals for each galaxy are always east or west, and is still proven to this day.
In the current year of 2.5 million ADE, humanity has colonized 14 galaxies with 9 galaxies west and 3 east. Galaxy 10 and Galaxy 4 haven't really been opened to the public and are still being occupied by military for the time being.
Wars are extinct, and capitalism dominates everything. Science and technology are near the peak of the 3rd dimension. Technology is no different from magic! Humanity is prospering!