Did these scrolls complete a parts of Christianity previously unknown? Who wrote them and why?
Not to forestall further answers or discussion, but you might be interested in /u/weinerdog73's answer to the question What do we know because of the Dead Sea Scrolls that we wouldn't have known otherwise?, which won one of our 'Best Of February 2017' awards.
I strongly recommend checking out the post that u/hillsonghoods posted since it greatly covers a lot about the dead sea scrolls and why they are significant. However, I did want to address one part of your question, just since there seems to be some confusion out there surrounding who wrote them and what communities they served.
First, I want to clear up your initial question. Christianity has been practiced since the presumed death of its founder Jesus of Nazareth probably around the year 30 -- however, it grew with historical figure of Paul of Tarsis, who started writing to Christians around the year 45 CE. This means that Christianity is between 1 - 2 decades shy of being practiced for 2,000 years, not 1,000 years. Another important fact for clarification is around the authorship of the documents you're referencing. The Dead Sea Scrolls were not written by Christians or for Christians. They were written between 200 BCE and 100 CE, meaning many of the surviving documents were actually written before Jesus of Nazareth ever existed. Archeologists and historians agree that they were written by a Jewish sect known as the Essenes. There is no evidence that the Essenes were influenced by first century Christianity in any way. They were deeply Jewish, and belonged to a very strict and passionate form of the faith, as historian Bart Ehrman explained:
The Essenes are the one Jewish sect not explicitly mentioned in the New Testament. Ironically, they are also the group about which we are best informed. This is because the famous Dead Sea Scrolls were evidently produced by a group of Essenes, who lived in a community east of Jerusalem in the wilderness area near the western shore of the Dead Sea, in a place that is today called Qumran. I say that they “evidently” produced the Scrolls because the term “Essene” never occurs in them. But we know from other ancient authors such as Josephus that a community of Essenes was located in this area; moreover, the social arrangements and theological views described in the Dead Sea Scrolls correspond to what we know about the Essenes from these other accounts. Most scholars are reasonably certain, therefore, that the Scrolls represent a library used by this sect, or at least by the part of it living near Qumran. (1)
As you can see, the Essenes had literally nothing to do with Christianity, especially since Christian texts didn't even mention the Essenes. u/weinerdog73's answers why the documents are significant, so I won't repeat everything here since that answer is great. If you have any follow ups, please feel free to post them here. I'm flaired in American religion, but I studied early Christianity while an undergrad, and did study the Dead Sea Scrolls as well.