in short 东西 is the informal version of 事物 which is very formal and rarely used. Often 事物 is used in cases you want to make an official statement or express abstract ideas such as physical laws. 外交方面的事物由大使馆代理。

这个东西很贵。有些事物并不是像它看起来的那样的，一个比较口语话，一个比较正式