The entrance to the Nebula Area
Bass in the Nebula Area
Nebula Area (ネビュラホールエリア Nebyura Hōru Eria, Nebula Hole Area in Japan), also known as Murkland (闇の世界 Yami no Sekai, "Dark World" in Japan), is a location in Mega Man Battle Network 5. The Nebula Area is the secret area of the game and is where all the negative energy goes. It is a 6-part area and in the last area, the negative energy forms the Chaos Lord. It can be unlocked by talking to a golden Mr. Prog present in Undernet after clearing the game once and having 100 Standard Battle Chips. Opening this area will cause stronger viruses to appear all over the world due to its dark power, but the entrance can be opened and closed by talking with the Mr. Prog, returning outside viruses to normal.
MegaMan has to complete three Liberation Missions to get to the back of the area. The first is directed by ShadeMan, the second by CloudMan and the last by both CosmoMan and BlizzardMan. There are virtually no NetNavis in Nebula Area except for a NetDealer with expensive and rare goods. The area also contains Dark Souls.
Dark Galaxy
A black hole to the Dark Galaxy
Dark Galaxy (あんこくせいうん ankoku seiun, dark nebula in Japan) is an unseen dark universe created by Dr. Regal that expands with Dark Power, with Nebula Gray being a dark star at its center. In the Dark Chip Factory, the Darkloids open a powerful Black Hole to the Dark Galaxy, sucking several NetNavis into it. It was possibly destroyed alongside Nebula Gray.
While Dark Galaxy appears to be the same place as Nebula Area, it is unknown if they are related, as the Dark Galaxy was specifically created by Regal and contains Nebula Gray as its core, while the Nebula Area, despite being named after Regal's organization, is implied to have come into existence due to the gathering of large amounts of Dark Power and contains the Chaos Lord. This can be compared with the bug beasts, Gregar being created naturally and Gospel by a human. Regardless of them being related or not, Nebula had great influence on it, Darkloids from Nebula reviving on it due to the Dark Power and working for the Chaos Lord.
Dark World/Murkland
While only accessed in Mega Man Battle Network 5, a Dark World was first mentioned in Mega Man Battle Network 3-4 in the Hole (Dark Hole in Japan) Battle Chip, which opens a gateway to a world of darkness and gives strength to Evil Chips. It is first translated as Murkland in Mega Man Battle Network 4, when Higsby informs that NetNavis that use Dark Chips are draw to it.
While not seen in the games, a world of darkness appears in the MegaMan NT Warrior manga as the home of the Darkloids, with Alpha opening a gate to its "DarkNet" and Colonel sealing the world with Giga Freeze.