[ngkabra]

Maybe the libraries are less known because they're lesser libraries?

---

"Less known libraries" would be libraries that fewer people know. "Lesser known libraries" would be libraries of a lower quality.

[source](http://english.stackexchange.com/questions/24719/difference-...)

[xapata]

You could hyphenate to clarify: lesser-known libraries.

But come on, the English language is ambiguous. We can figure out the meaning from the context. I'm usually a nit when it comes to vocabulary and grammar, but this one...

Actually, maybe the title should explain more of the purpose: Machine learning libraries that should be more popular.

[muktabh]

That's a grammatical mistake in the title. All the libraries mentioned have a of work behind them. [disclaimer: I helped compile the list]