The argument from ignorance (also known as ad ignorantium or appeal to ignorance) is the argument that a proposition is true because it has not yet been proven false or that it is false because it has not yet been proven true. The basic form of the argument is either:
P1. Statement A is not known (proved) to be true
C. Therefore it is false
P1. Statement A is not know (proved) to be false
C. Therefore it is true
"We do not know that there is life in the universe outside of that found on earth. Therefore, it does not exist."
This example can be seen as a fallacy of irrelevance in that the premise refers to our lack of knowledge which does not provide support for the substantive conclusion that there is no life outside of earth. The fact that we do not know one thing is not a relevant reason for believing another.
Yet there are occasions when a failure to find evidence can be seen as significant evidence in itself. For instance, if you were to search carefully in a small park for a bulldozer and fail to find one, you could argue:
"There is no evidence that there is a bulldozer in the park, therefore there is no bulldozer in the park."
The difference between the life outside of earth example and the bulldozer example has to do with what is sometimes called negative evidence. Negative evidence (or evidence of absence) is evidence that can be used to infer the non-existence or non-presence of something. The difference between the fallacious use of arguments from ignorance (an absence of evidence) and negative evidence (evidence of absence) can be nuanced and at times difficult to establish but in general it has to do with whether or not a careful, thorough search has been made. A bulldozer is a conspicuous object you'd expect to easily detect in a small city park if you conducted a search. The negative evidence of having conducted the search and not finding the bulldozer would be considered strong evidence that it is not there.
A Practical Study of Argument
Wikipedia: Evidence of absence
The Appeal to Ignorance or Argument Ad Ignorantiam