Raising capital without the actual app already built?

Id say right now you're outta luck. There's very little money going in the investment space (assuming you wanna raise from the USA). Right now most successful term sheets are given to people that are growing and revenue positive imo.

However, I think I would echo what @volkandkaya below, what is stopping you? With the amount of no-code options out there, you could easily set up a small MVP without too much time/financial investment. I would also recommend maybe using something like Figma/Sketch or any mockup tool to make mockups and put them on a landing page with the ability to have users sign up for your platform when it launches. Those are metrics you could take to a VC to help your case.

Finally, I would also look at angels (although angel investing has overall gone downhill recently). Angels are usually those who fund ideas that are this early in the stage. However, they are betting on you as a founder usually rather than the idea (which is bound to change an pivot often).

Not trying to be harsh, but if you ask this question, the answer is a flat no.

If you had a massively successful startup before, or even a massive growth, crash and burn maybe. But then you wouldn't ask that question here.

If you have an ivy league degree, solid idea, pitch deck and team in silicon valley, maybe. But you wouldn't be asking here.

For the 99% heavy funding in idea stage is not going to happen, though bootstrapping from idea to validation is far easier than ever now.

If you can't validate your idea yet, how do you validate investment?

Do you have customers? Or pre revenue? Or customers ready to use the product, verbal or contract commitments? (Assuming b2b)

I wouldn't rely on venture capital with the markets as they are right now. I'd double down on bootstrapping.

What is stopping you from building an MVP app?

If it is because you can't find a technical co-founder how do you plan on building a company that will scale to 100m+?