Researchers have found that roughly 11,000 entities are responsible for more than half of Bitcoin’s (BTC) on-chain volume.

According to a study published by the National Bureau of Economic Research (NBER) on Thursday, 11,043 on-chain entities represent 55% of the volume on the Bitcoin network. Cryptocurrency exchanges were estimated to account for three-quarters of on-chain volume.

The report found that the top 1,000-largest investors control roughly 3 million BTC or 15.9% of circulating Bitcoin, while the next 9,000-largest investors hold roughly 2 million BTC combined or 10.6% of circulating Bitcoin.

The report’s authors conclude that the network remains highly centralized despite the surge of new investors enticed by BTC’s 2021 bull market, stating:

“The Bitcoin ecosystem is still dominated by large and concentrated players, be it large miners, Bitcoin holders or exchanges.”

However, the study also noted that individual Bitcoin holders currently represent 8.5 million BTC or 45.1% of supply.

The NBER also identified significant concentration within the Bitcoin mining sector, estimating that 10% of the miners control 90% of the global hash rate. The report added that roughly 50 miners (approximately 0.1% of the network) command 50% of the Bitcoin network’s total hashing power.

While the NBER claims the centralization of the hash rate places the Bitcoin network at significant risk of a 51% attack, the report does not offer a hypothetical situation in which the world’s top miners would be incentivized to launch an attack on the network.

Related: Number of investors owning Bitcoin has tripled since 2018: Gallup Poll

According to Cambridge University’s Bitcoin Electricity Consumption Index (BECI), the global distribution of hash power has pluralized significantly since September 2019 — when China’s share peaked at 75.5%

While China’s renewed crackdown on domestic Bitcoin miners has been credited with driving a recent exodus of miners seeking cheap electricity in North America, Central Asia and Eastern Europe, the BECI’s data suggests that Chinese hashing power had already fallen by 40% before the April clampdown.