When you say the world I don’t know if you mean the earth or the universe but I’ll give seperate answers for both.
The current idea is that the universe started at the Big Bang, and expanded and cooled into the state we know today. We know this because when we look out into space we can measure the speed that things are moving by looking at the light pattern. When we do this we see that light from distant galaxies is nearly always shifted to the red end of the spectrum. This is because of the Doppler effect, where the wavelength of light changes depending on the speed of the object that it came from. If the light is shifted to the red side, it means the object is moving away from us. If this is happening everywhere it is natural to say that the whole universe itself must be expanding, like a balloon.
The Earth was formed from gases leftover from the formation of the Sun. The gases condensed and rocks started to come together because of gravity, then when the sun ignited it blew away most of the lighter gases, leaving behind the heavy rocks. Gravity brought these rocks together and until eventually they formed a planet.
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