The "middle ages" are anything between the collapse of Rome (400 AD) to its re-emergence in Rennaissance Italy (1400). Rennaissance means "rebirth". The "dark ages" are the period in the middle ages which were relatively undocumented. This would definitely include 400 AD to 700 AD, and some extend it to 900 AD.
Enter would create a new line, thus allowing you to enter a paragraph, on some applications you can use "Shift+Enter" if it is an IM application like Skype.
