"Deus ex machina" and "God of the Gaps"...
Both these expressions refer to the way in which God is often invoked as an explanation for things only because no other reasonable explanation can be found. "Deus ex machina" means "God out of the machine". Originally, it referred to a weak literary device in which an author would use some sudden and unrelated plot device to bring about a quick and speedy resolution to a crisis in the story (for example, the world is about to be destroyed by a massive meteorite, when suddenly—“dues ex machine”—some discovers the perfect weapon that’s been lying around just for this sort of occasion. Doctor Who is notorious for its dues ex machine plot resolutions, although they are a bit more forgivable in science fiction!)
“God of the gaps” is an expression which refers to the way in which people attribute things to God, just because no better explanation can be found. Wherever there is an absence of scientific information about a given event (such as the cause of the big bang) some theists are too quick to jump in with God as the solution to the cause for every unknown event.
The problem with assigning God a ‘deus ex machina’ or ‘God of the gaps’ role is that God is reduced to a plug for human ignorance rather than being a positive explanation. In other words, God becomes the cement that we use to plug up gaps in our knowledge of the universe. In time, as more and more scientific discoveries are made, God is replaced in these gaps by sound scientific analysis.
Any Argument for the existence of God, or an argument for or about religion, must avoid becoming an argumentum ad ignorantiam: an argument based on nothing more than our own ignorance. This sort of argument is always weak.