I want to know why this div won’t center horizontally until i specify this width

HTML

<div class="container">Dummy Text</div>

CSS

.container{
margin:auto;}

This doesn’t work but

.container{
margin:auto;
width: 20vw;}

this works.

question might seem silly, i started learning last week. thanks;
and i want to know why it isn’t working, i already know some workarounds;

i tried seeing through dev tools, but no luck. at first width is the width of the content. but it wouldn’t
work then.
but it starts working when i gave a specific width.