Could governments hide alien evidence forever?

It is very unlikely that any government could hide definitive evidence of alien life forever. A state might conceal a specific artifact, signal, or incident for a long time, but permanently suppressing something that important would face major practical, political, and technological limits.

Why long-term secrecy would be hard

• Independent observers exist. Private companies, universities, international observatories, and amateur skywatchers all have some ability to detect objects in space and orbit.
• No single government controls all data. Astronomical data, satellite tracking, and radio observations are spread across many institutions and countries, so one government would not fully control the evidence trail.
• Leaks and declassification happen. Large classified programs can stay secret for years, but over time the number of people involved, plus political turnover, makes exposure more likely.
• Politics creates incentives to disclose. If one country had solid proof, rivals would have reasons to verify it, expose it, or reveal it first.
• Records can resurface later. Files may be archived, reclassified, discovered through whistleblowers, or uncovered during government changes.

Where secrecy could still hold for a while

If the evidence were highly localized, ambiguous, or tightly controlled — for example, a single physical object in a secure facility, or a weak signal that only a small group can interpret — then secrecy could last decades. But “forever” is a much higher bar.

Bottom line

Governments could probably delay public knowledge of alien evidence, sometimes for a very long time, but hiding definitive evidence forever seems extremely unlikely.