• geart of matter,
• beart of matter,
• neart of matter,
• jeart of matter,
• ueart of matter,
• yeart of matter,
• hwart of matter,
• hsart of matter,
• hdart of matter,
• hrart of matter,
• h4art of matter,
• h3art of matter,
• hezrt of matter,
• hesrt of matter,
• hewrt of matter,
• heqrt of matter,
• heaet of matter,
• headt of matter,
• heaft of matter,
• heatt of matter

The adjective form of the phrase "heart of the matter" is "central" or "core". For example, if you want to describe something as related to the central or most important aspect, you could use the adjective "central" or "core".