The evidence is the examples in nature, especially those of human beings. If there was any great amount of evidence opposing these examples, there might be cause to think that complexity could arise on its own. But there isn't.
In other words, when we examine and correlate the intelligence behind the great numbers machines of intelligent man, plus the few simple machines of a few slightly intelligent animals, we see that intelligence makes machines, right? And the greater the intelligence, the greater the machines, right?
Which of the structures of any of the natural things on earth is not made up of machines? None of it. Even the complexities of subatomic particles reacting with each other is leverages upon leverages reacting with other leverages, both in material ways and in energy ways. This is what machines are! All, 100% of nature, is machines upon machines combined in a gigantic machine universe. And all, 100%, of our machines uses the examples of machinery found in nature, and the machines of nature themselves, one way or another in their design and operation.
Machines have makers. So, why would anyone think that the great machinery of nature, which has tremendously intelligent design to it, not have an Intelligent Designer behind it? We have no example of any intelligently designed "thing" that doesn't have an intelligent designer behind it. The closest we can come is to say that we don't know who the designer is.
Okay. You have figured out that cars and computers and nuclear power plants and ships and all the rest of the machines of mankind have come into being without intelligence, right? LOL! And mankind didn't use the machines of nature that are greater than theirs to figure all their stuff out, right? LOL! And mankind hasn't made all their machines out of the machines of natural subatomic particles in the least, right?
Nobody living saw the makers of many of the ancient machines of ancient countries like Egypt, Greece, and Rome. Consider the Antikythera mechanism - https://en.wikipedia.org/wiki/Antikythera_mechanism. Nobody knows who could have designed such a mechanism, to say nothing about knowing who did it. Are you trying to tell us that it simply grew under water on a sunken galleon? Yet it is made out of the machines of nature in the materials it is made out of. The bronze and wood out of which the mechanism is made are parts of the natural machinery.
Our examples of machinery that we know about show intelligence behind them. Machines, even the machines of nature that we use to make our machines, have makers. We see nothing else. Machines have makers.
Would a trillion man-made machines be enough? Would a trillion trillion? There is a point where the odds in a direction overcome the possibility of something in the opposite direction.
We don't know by direct observation where the machines of nature come from. But there is tremendous intelligence in the way it all works. Since we see multitudes of man-made, and even animal made machines that show intelligence, and nothing that we know of that shows the opposite (like a car appearing out in the desert out of thin air), there is an Intelligent Designer behind the design of the whole universe. No other way.