To reiterate what was said in a comment, that's not really possible, except in a way on mobile.
It is possible though to hide text as a spoiler. Like this (hover over it to show, click to fix it, on mobile one has to click but this is even indicated).
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The syntax is >! Text
That is sometimes used, but it is not really convenient as it takes up the full space on the site in standard view. However, on the mobile site it seems to be collapsed.
Possibly somebody could use MathJax in a clever way to about what you want. But that would be inconvenient and would have other drawbacks.
What I recommend instead is to format your posts in a good way. If it is really long, write a brief introduction, etc. There are various types of heading and ways to add emphasis. Judicious use of those will allow to present long post in a readable way.