There are a lot of interesting pages there.
There's a history outline.
It claims that the Emperor, the one true God, have created Mankind to be His chosen people, and have persided over it for most of it's history. Nothing else about the prehistory before year 1 is known, not even the event that started the calendar.
Years 1-15000 are known as the Age of Terra, and very little information is known about that age, but it is known that humankind have spread across the Galaxy and, under Emperor's watch, steadily progressed in it's technology.
Years 15000-25000 are called the Dark Age of Technology, in which the page claims humankind to have reached the peak of it's knowledge. Not much detail is given, but this quote is provided:
"In ancient times, men built wonders, laid claim to the stars and sought to better themselves for the good of all. But we are much wiser now."
- Archmagos Ultima Cryol
The article claims that the folly of ancient mankind, it's lust for knowledge deviating from Emperor's wishes, was exposed in the next age.
In around 25000-30000, The Age of Strife, the God-Emperor withdrew part of His omnipresent, pained protection over the wayward humanity. Witches and sorcerers started to emerge, and, unrestrained and unbound, laid waste to planets. Conflict over minute differences erupted across the now-unstable worlds, and a deadly series of machine uprisings led by Abominable Intelligences were just barely stopped. Most dramatically of all, at last, a series of increasingly severe warp storms erupted over the whole Galaxy, as the whole fabric of reality began to unravel, and the worlds of mankind became isolated and descended to various stages of barbarism.
It is then that the Emperor have divinely planned Mankind's salvation. And ultimately, at the end of the Age of Strife, He have conquered and rebuilt the old Terra* in a multiple-century series of campaigns now called the Unification Wars. He then created 20 sons, called Primarchs, each to lead a legion of Space Marines, the angelic warrior-monks at the spearhead of reconquest of Humanity. Unfortunately, a malicious Xeno plot have scattered the Primarchs across the Galaxy, and 2 of them were lost, and none of them were raised by God's perfect guidance.
In 30729, the Emperor's reborn Terra was united with the Machine Cult of Mars, and the Imperium was born, and the Great Crusade that would last two centuries would begin. Humankind was brought to unity, righteousness, and true faith by Emperor's Primarchs (found gradually along the way) and their legions of Space Marines, the Mechanicus-built Imperial Fleet the way for which was lit by Emperor's own Light, and the Imperial Guard; while xenos, mutants and heretics were struck down and psykers bound, reformed from witches and sorcerers to noble servants.
But then, two centuries later, the most trusted of the Primarchs, Horus, have betrayed his father, his commander-in-chief, and his God. In what is now known as the Horus Heresy, he have swayed half of the Primarchs and their Legions to his side by lies and sorcery, and burned down untold hundreds of worlds. The traitor besieged Terra, and used his corrupted demidivinity to slay dozens of saints and make a terrible blow upon the God-Emperor even as he was smitten into nothing in the process.
Yet, the article claims, the God-Emperor is indomitable, and did not truly die. His body sits immobile upon the Golden Throne, but his soul shines brighter than ever after his reascension to absolute divinity, still illuminating the way for Imperium's fleets and still saving the souls of His own righteous servants from damnation.
The aftermath of the Horus Heresy have seen Imperium beat the traitorous forces back into the warp storm known as Eye of Terror, and then slowly restore itself over around a thousand years in a period now known as The Time of Rebirth. The heretics will continue to launch pitiful offenses against Mankind, but all of them will fail.
Over the next period, the 2500 years of the Forging, the Imperium have firmly reentrenched itself in the Galaxy, while the true faith in the God-Emperor have faced numerous tribulations as the memory of the conquest began to wane, and multiple Wars of Faith have been declared upon various heretics, but in the end the truth have, of course, emerged victorious.
The times of troubles have, however, started. A 900 year period of interregnum with the Terra Nova faction was followed by an intensification of struggle between Administratum and Ecclesiarchy, and a subsequent takeover of the Ecclesiarchy by the then-acting Master of the Administratum, Goge Vandire, who then proceeded to subsume all other institutions of the Imperium under his sole command until his power was absolute.
His period of rule was called the Reign of Blood. It was, the article colorfully explains, an extremely stupid and extremely cruel tyranny.
(May I remind you that this is Imperium's official source?)
And when he was deposed, the Age of Apostasy continued with the Plague of Unbelief, a period where the trust in the one Imperium and one Ecclesiarchy was sabotaged but the previous (dictatorial and fanatical) mindset remained, so there were multiple rebellions and secessions led by false prophets.
This was followed by an Age of Redemption, Imperium's thousand-year-long bid to redeem itself by conquest that accomplished thousands of successes (unspecified).
And then comes the Present Age, in which humankind keeps vigil over Imperium's legacy, and slowly purifies itself in preparation for the prophesied End Times, when the Emperor finally heals himself, raises from the Golden Throne, vanquishes every remaining foe, and brings his people to a paradise in reality. Amen.