Based on its market cap, Apple (AAPL 0.17%) ranks as the biggest company in the world, valued at close to $2.8 trillion. It generated more than $383 billion in sales in its latest fiscal year and nearly $97 billion in profits. The company sits atop a cash stockpile of more than $162 billion.

All those big numbers would seem to imply Apple is doing great. There's just one glaring problem: Apple's overall sales aren't growing.

Don't assume this is a permanent problem, though. Actually, there's reason to believe it won't be. Apple CEO Tim Cook said six words in the company's recent earnings call that could point to a higher-growth future for the tech giant.

An intriguing question and a cryptic answer

In the conference call last week, Arete Research analyst Richard Kramer posed the last question -- and it was an intriguing one. Kramer first noted that Apple's annual research and development spending has increased by $8 billion over the past two years. He then asked what the main drivers behind the higher R&D investment were.

Kramer mentioned some possible areas for Apple's significant R&D in his question. He asked: "Is it Apple silicon? Is it new products like Vision Pro, or is it content to support new services?"

Cook confirmed that some of Apple's R&D spending increase over the last couple of years was indeed related to Vision Pro, the company's mixed-reality headset scheduled to launch in 2024. He also stated that some of the higher R&D amount was due to Apple's investment in its own silicon chips that are powering its new Mac computers.

There was also an area of heavier R&D costs that Kramer didn't list. Cook acknowledged that Apple is ramping up its investment in artificial intelligence (AI) and machine learning (ML). However, perhaps the most important part of his response was a cryptic answer of just six words: "Some things I can't talk about."

What might Apple be developing?

What might Apple be developing that Cook can't (or, more accurately, won't) talk about? I can think of five products off the top of my head that Apple is rumored to be working on.

First, it's been widely reported that Apple is developing its own chatbot that's been referred to as "Apple GPT" built on its Ajax large language model. However, since Cook specifically mentioned AI and ML in his answer about what's behind the company's increased R&D spending, we could potentially scratch this one off the list.

Another possibility is that Apple is building its own search engine. Some think this could be a smart move for the company that would significantly boost its advertising revenue. An argument against this theory, though, is that Alphabet will pay Apple a whopping $19 billion this year for Google Search to be the default search engine on the company's Safari browser, based on one analyst's estimate.

Apple is also rumored to be working on a self-driving electric car. There are ample reasons to believe these rumors are true. Some predict that the "Apple car" will be unveiled in 2026 and that its price tag will be below $100,000.

I think that augmented reality (AR) glasses could be another top candidate. Apple has made AR a big focus with Vision Pro. It wouldn't be surprising if the company uses some of the technology incorporated in its forthcoming mixed-reality headset in a lower-cost device as well.

Finally, there's no question that Apple is continuing to invest in R&D related to the iPhone. I'd put money on the company rolling out a folding version of its smartphone in the near future. There could be other innovations on the way as well.

What we don't know will probably help us

It wouldn't surprise me if Apple is developing all the products mentioned, plus others not on the list. We can't know for sure. But what we can know for sure is that Apple's management isn't spending money on R&D that it doesn't think will eventually pay off.

Take a look at the chart. Over the long term, Apple's revenue growth has tracked pretty well with its R&D investment growth.

AAPL Research and Development Expense (TTM) Chart

AAPL Research and Development Expense (TTM) data by YCharts

Now look at the more recent trend. Apple's revenue growth is much slower than its R&D growth.

AAPL Research and Development Expense (TTM) Chart

AAPL Research and Development Expense (TTM) data by YCharts

Over a longer time period, the company's revenue growth climbed at a similar trajectory as its R&D spending growth. I suspect that we'll see Apple's R&D investments begin to pay off in a bigger way in the coming years as history repeats itself.

In a sense, I'm glad that Tim Cook can't talk about some of the things that Apple is doing. For all of us Apple shareholders, what we don't know in this case will probably help us over the long term.