the law of conservation of matter says that matter has neither been created nor destroyed. But ,we know that all matters are created by the big bang explosion. So,is the law of conservation of matter wrong?

Dear Student ,
Big bang is a hypothesis , it is not a proven theory .Until it is proven we can not say that Big Bang has created all the mass .
And for your concern Conservation of matter is not wrong .Matter and energy are conserved quantities .We do not know from where does this matter came but we know that matter can be converted in to energy and energy can also convert itself to matter .

Hope this helps you .
Regards