BTC dominance measures the relative value of Bitcoin compared to all other altcoins and tokens combined. It is a ratio expressed as a percentage. For example '80%' BTC dominance means that Bitcoin holds 80% of all the value in cryptocurrencies, whilst alternative coins represent 20% of that total value.
For as long as I've been monitoring it (basically when coinmarketcap.com started including it on their site) the value for this metric has been maintained at over 80%, typically averaging ~82% with a surprisingly low level of deviation from that average. This lack of volatility compared to price volatility suggests to me that there is something important at play
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