"The presence of design in the universe proves there is a God. Surely you don't think all this appeared here just by chance?"
This is known as the Argument From Design.
It is a matter of dispute whether there is any element of design in the universe. Those who believe that the complexity and diversity of living creatures on the earth is evidence of a creator are advised to consult the Talk Origins Archive.
There is insufficient space to summarize both sides of that debate here. However, the conclusion is that there is no scientific evidence in favor of so-called Scientific Creationism. Furthermore, there is much evidence, observation and theory that can explain many of the complexities of the universe and life on earth.
The origin of the Argument by Design is a feeling that the existence of something as incredibly intricate as, say, a human is so improbable that surely it can't have come about by chance; that surely there must be some external intelligence directing things so that humans come from the chaos deliberately.
But if human intelligence is so improbable, surely the existence of a mind capable of fashioning an entire universe complete with conscious beings must be immeasurably more unlikely? The approach used to argue in favor of the existence of a creator can be turned around and applied to the Creationist position.
This leads us to the familiar theme of "If a creator created the universe, what created the creator?" but with the addition of spiraling improbability. The only way out is to declare that the creator was not created and just "is" (or "was").
From here we might as well ask what is wrong with saying that the universe just "is" without introducing a creator? Indeed Stephen Hawking, in his book "A Brief History of Time," explains his theory that the universe is closed and finite in extent, with no beginning or end.
The Argument From Design is often stated by analogy, in the so-called Watchmaker Argument. One is asked to imagine that one has found a watch on the beach. Does one assume that it was created by a watchmaker, or that it evolved naturally? Of course one assumes a watchmaker. Yet like the watch, the universe is intricate and complex; so, the argument goes, the universe too must have a creator.
The Watchmaker analogy suffers from three particular flaws, over and above those common to all Arguments By Design. Firstly, a watchmaker creates watches from pre-existing materials, whereas God is claimed to have created the universe from nothing. These two sorts of creation are clearly fundamentally different, and the analogy is therefore rather weak.
Secondly, a watchmaker makes watches, but there are many other things in the world. If we walked further along the beach and found a nuclear reactor, we wouldn't assume it was created by the watchmaker. The argument would therefore suggest a multitude of creators, each responsible for a different part of creation (or a different universe, if you allow the possibility that there might be more than one).
Finally, in the first part of the watchmaker argument we conclude that the watch is not part of nature because it is ordered, and therefore stands out from the randomness of nature. Yet in the second part of the argument, we start from the position that the universe is obviously not random, but shows elements of order. The Watchmaker argument is thus internally inconsistent.
Apart from logical inconsistencies in the watchmaker argument, it's worth pointing out that biological systems and mechanical systems behave very differently. What's unlikely for a pile of gears is not necessarily unlikely for a mixture of biological molecules.