人们发现河外星系有多少个？

How many does people discover the external galaxy having?

我们用肉眼是看不到河外星系的。

Extragalactic nebula is invisible to the naked eye.

整个宇宙拥有十亿多个河外星系，一个河外星系就是一座星城，每座星城有“居民”上千亿颗。

Our Universe contains over a billion galaxies; star cities each with a hundred billion inhabitants.