Void Century

The Void Century (空白の歴史, Kūhaku no Rekishi; lit. Blank Chronicle), also referred to as the Blank Century, was hinted at during the Skypiea Arc; the dates were seen in Robin's flashback. The years of the void century range from 600 AOS-700 AOS in the Age of the Sea.

It is a century long gap in recorded and archaeological history, the study of which is forbidden by the World Government.

About
This period became known as the blank century because there is nothing left to acknowledge anything that happened, aside from the Poneglyphs which are written in a language that few know how to read.

However, it causes a problem with the known histories of the world as nothing ties up because of that missing period, although most people do not seem to be aware about this information. For example, Nefertari Cobra after being told by Nico Robin about this missing period was completely shocked by its existence and didn't know there were parts of the world history that didn't tie in.

The Great War
The most major event during the Void century seems to be the Great War.

Moon
In Enel's cover story, there is an ancient city on the moon with another bunch of identical Spaceys. There is a painting on the wall that shows a Skypiean with Spaceys following him on the left while a Shandorian and Bilkan work on the right. After resources on the moon ran out, the three races departed for the "Blue Star".

One of the three races, the Shandorians, ended up on the isle of Jaya where they built the city of Shandora 1000 years ago. The great city of Shandora collapsed during this war, the Shandorians defended the city against those "who seek the stone". The city collapsed but the Shandians, who are descendants from the Shandorians, kept on defending the city for over 800 years afterwards.

All three races are noted to have wings on their backs (though each has a slightly different style) and may have even once been the same race.

Interestingly, near the end of the Skypeia arc, after Montblanc Cricket confirmed the existence of the city of Gold he said that the Sky Island has an "old and boring history". Another connection beetween the sky island of Skypeia and the void century is the Poneglyph left by Gold Roger there.

Ancient Kingdom
The Ancient Kingdom was a giant and powerful kingdom that was destroyed by the Twenty Kingdoms, which later would evolve into the World Government.

Birth of the World Government
In 700, at the end of the Void Century, the World Government was born and took political control of the entire world, uniting all countries and forming the Counsel of Kings. For the World Government, the events of the void century were better left unknown as information linked to it is considered far too dangerous.

When the research of the ponegylphs was outlawed, hundreds of scholars across the world were killed. Since then, a ban on any research to the void century has been put in place.

How the World Government was founded is known. However, how they manage to cover up 100 missing years, and how the World government came to be what it is now, remained a secret.

Poneglyph
Poneglyphs are mysterious stone blocks with history inscribed onto it, and these blocks are scattered all over the Grand Line. The Government has forbidden anyone to research on these blocks.

Ancient Weapons
Only 2 weapons are mentioned but were never shown. The Ancient Weapons known in the story so far are Pluton and Poseidon. Each weapon was constructed during the void century and so far all mentioned weapons were capable of extreme mass destruction, such as the complete destruction of entire islands.

The Ancient Weapons is the argument used by the World Government to forbid the research of the missing years. And at least this seems to be true, as Shiki said that Gold Roger knew the Ancient Weapons.

Trivia

 * Gold Roger's crew appears to have followed the Poneglyphs all the way and figured out exactly what the missing history is. When queried by Nico Robin on the blank history and the Will of D, Rayleigh replied to her that he would tell her the history if she wanted to know. At the same time, he appeared reluctant to even speak about it. He warned her if she found out now about the history, in her current condition, she wouldn't be able to do a thing about it. In his closing statement, he explained that he believed his crew and the scholars of Ohara had been too hasty. At the same time, he encouraged her to seek it out for herself on the grounds that she might come to a different conclusion than the one his crew had.